There seems to be some confusion about "joining" groups.
You don't need to "join" a group to participate in its diaries. You can comment and recommend any diary, from any group or individual, just like you've always been able to.
Joining a group means becoming an admin for the group. That's by invitation.
Following the group merely funnels its diaries into your stream. That's optional, if you never want to miss a group's diaries.
So there's the crowd that plans the party, the crowd that signs up for the invite list, so they know when and where the party is happening, and then there are those who merely show up when they come across the party.
So to be clear, you only join a group if you're going to help run it. For 99.99% of you, that's not anything you need to worry about.