I used to do a "girl" group. It was middle school girls having friendship issues (bullying, mean girl stuff). It was pretty successful.
I also did a group for elementary aged boys for anger management. It was ok, but slightly less successful
I did a group for 1st and 2nd grade boys- friendship was the topic. Successful.
I taught a "gifted" group and it was semi-successful.
I did a few social skills groups for various ages. Successfulness varied.
All of these groups happened at lunch/recess times, so it was hard to actually feel like I was doing a real "group" because we spent time eating and dealing with constraints, and some kids were more reluctant to spend recess in the group, although we tried to make it fun.
I wish I could have had groups at other times, but it's hard to take away from instructional time. I get it. I've taught (like I think we all have?).