I think from a listeners point of view the idea is you can go to a groups main page and set it playing track by track, so the idea I believe is that you could check out say, a tech house group, and there would only be those kind of tunes in there.
From a contributors point of view you can put your stuff into groups for your genre, some are moderated (i.e. someone vets your work belongs to the group/genre) and some are just free for all. You can contribute by joining a group and then on your track there's an 'add to group' button.
Is it worth it ? hrmmmm..... it's minimal effort really but you don't get much back either, don't think many people really use soundcloud in this way (well i don't anyway, I follow stuff I like and then use my stream). I tried submitting some tracks to groups and get the occasional random listen I think, but the downside it people use them as an easy way to spam for followers for their type of music (you know, the kind that unfollow after about a week and have 2000 people they are following). Overall really it's probably not worth the bother, but like I say - it's pretty minimal effort. The moderated groups are probably the best (as those would be the ones I'd listen to I guess), but they can take time to approve your tune etc.