I think the gym club is a great idea because it's based on a universal topic that can't be exhausted. It's an ongoing theme of the world (such as video gamers, LGBT, atheist etc) so it'll be easy to keep rolling with it. The thing I've noticed about fan clubs of specific things like TV or book series though, is that they tend to die very quickly - so with the exception of Harry Potter, I tend to stay away from them. I think with TV shows, as much as we all love them and share in the joy, there's only so much that can be said about them (particularly comedies like HIMYM and Community). So if you're going to make a club on either of those, I'd make sure you have a lot of people who would be committed to helping you keep them alive.
I'd join the HIMYM Club because I watch and love the show, but I think it'd be hard to keep up the momentum because the discussion is dealing with a finite source material.
WOW I SOUND LIKE SUCH A BUZZKILL. I don't mean to be discouraging haha, that's just my observations from being in Other Clubs for so long XD