I don't, but I'd actually like to. I'm big on artists helping out other artists, but I don't know enough good groups. I see a lot of new groups that are soon to die or groups so saturated with members that you know they're either picky, or upload so much daily that it become white noise.
I've had one of my pieces submitted by someone else to one group, but DA asked for my permission before it was posted. If submitting is like that for everyone (permission is asked for), then I don't see why not. You can just disapprove of the submission if you don't want it.
Huh, I didn't actually know what they were before looking them up - now I know there's a bunch of different ones of all themes, which is pretty cool. I like the idea of having a little club and nominating another person's bit of art to it. =O Coooool.
Now I need to join a group? AAAHHH GROUP JOINING VIRGINITY. I HAS IT.
All the time! I'm always looking for exceptional art myself, so if I see apiece of art that is really really good but has very little exposure I'll definitely submit it to one of the groups so more people can have a chance to see it. In fact, only accepts artwork submitted (I like to think of it as "nominated") by a fellow deviant.
I suggest art others art to galleries of groups I joined all the time. If it's a wonderful piece it deserves more views. The artist needs to approve it being added to the group anyway so it's not like you are submitting it against their will.