diff --git a/webapp/components/FollowButton.spec.js b/webapp/components/Button/FollowButton.spec.js
similarity index 100%
rename from webapp/components/FollowButton.spec.js
rename to webapp/components/Button/FollowButton.spec.js
diff --git a/webapp/components/FollowButton.vue b/webapp/components/Button/FollowButton.vue
similarity index 99%
rename from webapp/components/FollowButton.vue
rename to webapp/components/Button/FollowButton.vue
index 126a3891d..a0807ed6c 100644
--- a/webapp/components/FollowButton.vue
+++ b/webapp/components/Button/FollowButton.vue
@@ -19,7 +19,6 @@ import { followUserMutation, unfollowUserMutation } from '~/graphql/User'
 
 export default {
   name: 'HcFollowButton',
-
   props: {
     followId: { type: String, default: null },
     isFollowed: { type: Boolean, default: false },
diff --git a/webapp/components/Button/JoinLeaveButton.vue b/webapp/components/Button/JoinLeaveButton.vue
new file mode 100644
index 000000000..bb284dbef
--- /dev/null
+++ b/webapp/components/Button/JoinLeaveButton.vue
@@ -0,0 +1,91 @@
+<template>
+  <base-button
+    class="track-button"
+    :disabled="disabled"
+    :loading="loading"
+    :icon="icon"
+    :filled="isMember && !hovered"
+    :danger="isMember && hovered"
+    @mouseenter.native="onHover"
+    @mouseleave.native="hovered = false"
+    @click.prevent="toggle"
+  >
+    {{ label }}
+  </base-button>
+</template>
+
+<script>
+import { joinGroupMutation } from '~/graphql/groups'
+
+export default {
+  name: 'JoinLeaveButton',
+  props: {
+    groupId: { type: String, required: true },
+    userId: { type: String, required: true },
+    isMember: { type: Boolean, required: true },
+  },
+  data() {
+    return {
+      disabled: false,
+      loading: false,
+      hovered: false,
+    }
+  },
+  computed: {
+    icon() {
+      if (this.isMember && this.hovered) {
+        return 'close'
+      } else {
+        return this.isMember ? 'check' : 'plus'
+      }
+    },
+    label() {
+      if (this.isMember) {
+        return this.$t('group.joinLeaveButton.iAmMember')
+      } else {
+        return this.$t('group.joinLeaveButton.join')
+      }
+    },
+  },
+  watch: {
+    isMember() {
+      this.loading = false
+      this.hovered = false
+    },
+  },
+  methods: {
+    onHover() {
+      if (!this.disabled && !this.loading) {
+        this.hovered = true
+      }
+    },
+    async toggle() {
+      const join = !this.isMember
+      const mutation = join ? joinGroupMutation : null // Wolle: implement "leaveGroupMutation"
+
+      this.hovered = false
+      const optimisticResult = { joinedByCurrentUser: join }
+      this.$emit('optimistic', optimisticResult)
+
+      try {
+        const { data } = await this.$apollo.mutate({
+          mutation,
+          variables: { groupId: this.groupId, userId: this.userId },
+        })
+        const joinedGroup = join ? data.JoinGroup : { myRoleInGroup: null } // Wolle: implement "leaveGroupMutation"
+        this.$emit('update', joinedGroup)
+      } catch (error) {
+        optimisticResult.joinedByCurrentUser = !join
+        this.$emit('optimistic', optimisticResult)
+      }
+    },
+  },
+}
+</script>
+
+<style lang="scss">
+.track-button {
+  display: block;
+  width: 100%;
+}
+</style>
diff --git a/webapp/locales/de.json b/webapp/locales/de.json
index 86602a7c1..1fc4e3fa7 100644
--- a/webapp/locales/de.json
+++ b/webapp/locales/de.json
@@ -373,6 +373,10 @@
   "group": {
     "foundation": "Gründung",
     "goal": "Ziel:",
+    "joinLeaveButton": {
+      "iAmMember": "Bin Mitglied",
+      "join": "Beitreten"
+    },
     "membersCount": "Mitglieder",
     "membersListTitle": "Gruppenmitglieder"
   },
diff --git a/webapp/locales/en.json b/webapp/locales/en.json
index 7e0def985..b1a9356a9 100644
--- a/webapp/locales/en.json
+++ b/webapp/locales/en.json
@@ -373,6 +373,10 @@
   "group": {
     "foundation": "Foundation",
     "goal": "Goal:",
+    "joinLeaveButton": {
+      "iAmMember": "I'm a member",
+      "join": "Join"
+    },
     "membersCount": "Members",
     "membersListTitle": "Group Members"
   },
diff --git a/webapp/locales/es.json b/webapp/locales/es.json
index 791874729..63070ee4b 100644
--- a/webapp/locales/es.json
+++ b/webapp/locales/es.json
@@ -300,6 +300,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/fr.json b/webapp/locales/fr.json
index 66675942c..91b91abe9 100644
--- a/webapp/locales/fr.json
+++ b/webapp/locales/fr.json
@@ -289,6 +289,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/it.json b/webapp/locales/it.json
index c9ff7870b..342550a14 100644
--- a/webapp/locales/it.json
+++ b/webapp/locales/it.json
@@ -297,6 +297,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/nl.json b/webapp/locales/nl.json
index 02039e0db..c7e474a3a 100644
--- a/webapp/locales/nl.json
+++ b/webapp/locales/nl.json
@@ -85,6 +85,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/pl.json b/webapp/locales/pl.json
index c371a8831..840698487 100644
--- a/webapp/locales/pl.json
+++ b/webapp/locales/pl.json
@@ -169,6 +169,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/pt.json b/webapp/locales/pt.json
index fe419f053..bec922edb 100644
--- a/webapp/locales/pt.json
+++ b/webapp/locales/pt.json
@@ -335,6 +335,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/locales/ru.json b/webapp/locales/ru.json
index bb0f34086..f50121635 100644
--- a/webapp/locales/ru.json
+++ b/webapp/locales/ru.json
@@ -314,6 +314,10 @@
   "group": {
     "foundation": null,
     "goal": null,
+    "joinLeaveButton": {
+      "iAmMember": null,
+      "join": null
+    },
     "membersCount": null,
     "membersListTitle": null
   },
diff --git a/webapp/pages/group/_id/_slug.vue b/webapp/pages/group/_id/_slug.vue
index 2011e0a8c..f21a71942 100644
--- a/webapp/pages/group/_id/_slug.vue
+++ b/webapp/pages/group/_id/_slug.vue
@@ -49,7 +49,11 @@
             <ds-flex-item>
               <client-only>
                 <ds-number :label="$t('group.membersCount')">
-                  <count-to slot="count" :start-val="0" :end-val="groupMembers.length" />
+                  <count-to
+                    slot="count"
+                    :start-val="membersCountStartValue"
+                    :end-val="groupMembers.length"
+                  />
                 </ds-number>
               </client-only>
             </ds-flex-item>
@@ -72,7 +76,7 @@
               </client-only>
             </ds-flex-item> -->
           </ds-flex>
-          <div v-if="!isGroupMember" class="action-buttons">
+          <div class="action-buttons">
             <!-- <base-button v-if="user.isBlocked" @click="unblockUser(user)">
               {{ $t('settings.blocked-users.unblock') }}
             </base-button>
@@ -86,6 +90,15 @@
               @optimistic="optimisticFollow"
               @update="updateFollow"
             /> -->
+            <join-leave-button
+              :groupId="group ? group.id : ''"
+              :userId="currentUser.id"
+              :isMember="isGroupMember"
+              @optimistic="optimisticJoinLeave"
+              @update="updateJoinLeave"
+            />
+            <!-- implement:
+              v-if="!user.isMuted && !user.isBlocked" -->
           </div>
           <template v-if="group.about">
             <hr />
@@ -200,8 +213,9 @@ import AvatarUploader from '~/components/Uploader/AvatarUploader'
 // import ContentMenu from '~/components/ContentMenu/ContentMenu'
 import CountTo from '~/components/CountTo.vue'
 import Empty from '~/components/Empty/Empty'
-// import FollowButton from '~/components/FollowButton.vue'
+// import FollowButton from '~/components/Button/FollowButton'
 // import FollowList from '~/components/features/ProfileList/FollowList'
+import JoinLeaveButton from '~/components/Button/JoinLeaveButton'
 import MasonryGrid from '~/components/MasonryGrid/MasonryGrid.vue'
 import MasonryGridItem from '~/components/MasonryGrid/MasonryGridItem.vue'
 import PostTeaser from '~/components/PostTeaser/PostTeaser.vue'
@@ -226,6 +240,7 @@ export default {
     Empty,
     // FollowButton,
     // FollowList,
+    JoinLeaveButton,
     PostTeaser,
     ProfileAvatar,
     ProfileList,
@@ -253,22 +268,26 @@ export default {
       // followedByCountStartValue: 0,
       // followedByCount: 7,
       // followingCount: 7,
-      membersCount: Infinity,
+      membersCountStartValue: 0,
+      membersCountToLoad: Infinity,
       updateGroupMutation,
     }
   },
   computed: {
+    currentUser() {
+      return this.$store.getters['auth/user']
+    },
     group() {
-      return this.Group ? this.Group[0] : {}
+      return this.Group[0] ? this.Group[0] : {}
     },
     groupMembers() {
       return this.GroupMembers ? this.GroupMembers : []
     },
     isGroupOwner() {
-      return this.group.myRole === 'owner'
+      return this.group ? this.group.myRole === 'owner' : false
     },
     isGroupMember() {
-      return this.group.myRole
+      return this.group ? !!this.group.myRole : false
     },
     groupName() {
       const { name } = this.group || {}
@@ -405,8 +424,32 @@ export default {
     //   this.user.followedByCurrentUser = followedByCurrentUser
     //   this.user.followedBy = followedBy
     // },
+    optimisticJoinLeave({ joinedByCurrentUser }) {
+      /*
+       * Note: "membersCountStartValue" is updated to avoid counting from 0 when join/leave
+       */
+      this.membersCountStartValue = this.GroupMembers.length
+      if (joinedByCurrentUser) {
+        // this.membersCountToLoad++
+        this.GroupMembers = [this.currentUser, ...this.GroupMembers]
+      } else {
+        // this.membersCountToLoad--
+        this.GroupMembers = this.GroupMembers.filter((user) => user.id !== this.currentUser.id)
+      }
+    },
+    updateJoinLeave({ myRoleInGroup }) {
+      this.Group = [{ ...this.Group[0], myRole: myRoleInGroup }] // if we assign it directly "this.group" will not be updated
+      const currentUserInGroupMembers = this.GroupMembers.find(
+        (user) => user.id === this.currentUser.id,
+      )
+      if (currentUserInGroupMembers) {
+        currentUserInGroupMembers.myRoleInGroup = myRoleInGroup
+      } else {
+        this.$apollo.queries.GroupMembers.refetch()
+      }
+    },
     fetchAllMembers() {
-      this.membersCount = Infinity
+      this.membersCountToLoad = Infinity
     },
   },
   apollo: {
diff --git a/webapp/pages/profile/_id/_slug.vue b/webapp/pages/profile/_id/_slug.vue
index 7b091c297..0b3212a0b 100644
--- a/webapp/pages/profile/_id/_slug.vue
+++ b/webapp/pages/profile/_id/_slug.vue
@@ -172,7 +172,7 @@
 import uniqBy from 'lodash/uniqBy'
 import postListActions from '~/mixins/postListActions'
 import PostTeaser from '~/components/PostTeaser/PostTeaser.vue'
-import HcFollowButton from '~/components/FollowButton.vue'
+import HcFollowButton from '~/components/Button/FollowButton'
 import HcCountTo from '~/components/CountTo.vue'
 import HcBadges from '~/components/Badges.vue'
 import FollowList, { followListVisibleCount } from '~/components/features/ProfileList/FollowList'