It seems the answer I gave was deleted from the server for "Group Setting (Membership Requests - Auto-accept requests?)"
I have resubmitted my answer here since the thread is locked. This isn't the best answer but it is what we are doing.
***
Set up your group and under Membership Requests select Yes on both questions (Allow requests to join/leave this group? AND Auto-accept requests?).
From the group page get the url.
Set up a page (I used a wiki) with user instructions and create links to the group pages.
Users must Click the drop down on Actions and then click either Join or Leave Group.
I would like to give them links directly to join or leave but haven't figured that out yet.