Why not teach him about taking turns. Let's be frank now, playgroups are not about sharing. I stopped going with ds at around 2 as they were horrid IMO! Hard to teach sharing when a kid runs off with what he was playing with.
He obviously knows what to do when not at playgroup so I wouldn't worry.
2.5 DS - crap at sharing Plays nicely with siblings and children he knows Playgroup type scenario, forget it - he's horrendous Thought a book might help My other 2 were always alright with this sort of thing