Fix component to add a member

src/components/Memberships/AddMemberForm.vue

@@ -129,7 +129,7 @@ export default {
   methods: {
     ...mapMutations('notifications', ['notify']),
     async add () {
-      if (this.loading || !isAdmin) return
+      if (this.loading || !this.isAdmin) return
 
       const identifier = this.fields.identifier.trim()
       const level = this.fields.level