Hi everyone. I'm mulling some ideas for a website I'm designing (weight loss related). While I intend for it to be a community site of sorts, I'm debating which would be better: having a forum, or having community groups. I'm reluctant to choose both- I don't want to split my time in too many directions, and I don't want to start the site off with too many website features.

First, I would prefer forums but several facts have me second-guessing myself:
1) Websites of similar niches have community groups. While I don't want to copy other sites, if their target demographic is the same as mine, would those website users EXPECT to have groups available?
2) Groups double as a even planning utility for users who want to organize outings, get-togethers and meetings in the real world.
3) Users tend to have more control over their groups than they would forums and they may like this kind of control.

On the other hand, I tend to see groups as glorified forums with a different UI. If a group is just a forum with a slightly different look, I'm leaning towards sticking with a well-themed forum.

I'd love to hear any of your experiences with using forums vs. groups. Are there some situations where one works better than the other for community sites?