I have set a group to ‘Private’. When a non-logged-in

I have set a group to ‘Private’. When a non-logged-in user tries to access the forum, they correctly get, “This is a private group. To join you must be a registered site member and request group membership.”

HOWEVER, there is no ‘REQUEST MEMBERSHIP’ button (on this page, or anywhere else). I didn’t think I needed a plugin. Am I wrong? If so, what plugin do you suggest for such a simple thing (I am not collecting money from anyone so don’t require anything like S2Member).