There are two broad classes of membership: Free and Paid-for. Whereas a paid-for subscription allows you access to the whole site, a Free one only gives you access to out-of-copyright music.
If you already have paid-for membership, then you already have full access, so there is no need to become a member of a group.
Once your paid-for membership expires, you can either “renew” your membership or, if you want to join a group, “upgrade or change” your membership to the Free one; you will then be allowed to join a group.
If you are already a member of a group then, again, you already have full access.