Redirect to group members tab if user cannot see messages tab.

app/assets/javascripts/discourse/routes/group-messages.js.es6

@@ -2,4 +2,14 @@ export default Discourse.Route.extend({
   titleToken() {
     return I18n.t('groups.messages');
   },
+
+  model() {
+    return this.modelFor('group');
+  },
+
+  afterModel(group) {
+    if (!group.get('is_group_user') || !(this.currentUser && this.currentUser.admin)) {
+      this.transitionTo("group.members", group);
+    }
+  },
 });