"Sorry, you are unable to join this group. You are already an administrator of three or more groups. Please leave any other group, or upgrade to Premium Membership first."
People signing up to be members will actually be signing up to be founder, co-founder, or contributor it seems. Not sure which one.
In the Manage Members area, click on "Members" and adjust the join requests there. That should fix things.(Folder permissions are also accessed this way.)