J.K.
I can relate, My 3 yr old can be that way.
I know it can be so embarrassing to have the kid who hits or bullys at playgroup. But, It is important to keep exposing him to these situations.
I have found that it helps if I spell out the rules prior to going. In the car we say "NO Hitting" "No Screaming" etc.
I also try to give her the correct behavior. No hitting-gentle or kind touches. No Screaming-inside voice or quiet voice.
We also made a family rule poster and believe me my dd lets me know if I broke any rules (ie NO yelling, Hey I am a mom)
HOw is his vocabulary? He may be acting out of frustration and trying to get someone to listen to him.
I would also keep a close eye on him and stay close to him at the beginning of playgroup to intervene before things go to blows. I know you want the adult interaction but, explain to other mom's I want him to learn good manners and how to play correctly around friends. I am sure it will pay off in the long run.