I've tried to join the Classic Rock coalition - are these groups solely for CDLC creators?  I don't seem to have administrative rights to get in there.  Strange.

You may have to be approved by the moderators of the group (the user's who created the group). not sure how it all works yet myself.

I don't think the group consists solely of CDLC creators. While I guess it's main purpose is making CDLC, I think they have some people who test charts others make but don't make any themselves. Someone from the group like @@avdocatwork could probably answer the question more definitively.

there should be a "join the group" button at the group main page since members have to be approved by mod. Just push it and wait. Otherwise send a pm to  @avdocatwork

How am I supposed to know which ones I might want to join?  I don't see any...

At top - OTHER -> SOCIAL GROUPS. There you can choose which one you want to join. And after going to their page, there is "Join Group" button.