No, I don't have an update. Kristine, from Brooklyn, NY, if you're out there, fill us in.
I get how stressful this can be for you. Part of the reason he might keep repeating the action is because he gets a sense of power from doing it; he sees that it affects you. Remind yourself: this is a phase, it will pass. The more matter of fact you can be in your reaction, the quicker it will disappear. Oh -- and consistency counts. You need to have the same exact reaction each time it happens. Part of what motivates young children is cause and effect: If I do this, what happens? What about this time? What about now? Your son needs to see, over and over, that hair-pulling results in him getting cut off from the people/activity he is enjoying.
I wouldn't worry so much about being asked to leave the playgroup. Every parent, and facilitator, knows that this kind of behavior turns on a dime. The kid who is the victim today could be the perpetrator tomorrow. It doesn't forecast future behavior or broadcast behavioral characteristics. It's just a developmental stage that some children go through on the way to gaining language.
I'm repeating here what I wrote previously:
Try to anticipate when it will happen. He probably pulls another child's hair for one of three reasons: He wants what the child has; he wants that child's attention, often to stop the action; or he wants an adult's attention and he knows this gets a reaction. If you can see it coming, intervene and give him the words he needs: "You want to play with the truck that Justin has. Let's ask Justin if you can have a turn." If you actually see his little arm moving into position to pull and it's logistically possible, gently grab his arm and tell him, "No! We don't pull hair." And then again, try to identify and label what he's feeling.
When he succeeds in pulling someone's hair, immediately pick him up and remove him from the activity. Say, "No hair pulling!" And then: "You can't play if you pull hair." Removing him from the action is powerful. He'll be unhappy. Try to engage him in something else for a few minutes (seconds?) and then tell him, "Can you play with Justin without pulling hair? Let's try." If it happens again, remove him again.
Alternatively, if it is your hair he pulls, get up and walk away from him, or turn your back on him. Tell him, "I can't play when you pull my hair. Tell me when you are ready to try to play again." If he does it again, repeat the same response as many times as necessary. Stay calm, matter-of-fact, and firm. He'll get he message. Toddlers learn by repetition. Sometimes it just takes a lot of it.
Here's what you never want to do: Pull his hair. I'm always hearing about parents who do that, thinking their kiddo will see how it hurts and stop. It doesn't compute that way. A child interprets it that if mom does it, it's OK for me, too.