Well one of the Dotsie and Sadie lookalikes is Rentawitch from this site. The guy with the pipe is Bernard Pearson (The Cunning Artificer), the Miss Treason is Jacqueline Simpson, the author of The Folklore of Discworld. The Cohen the Barbarian's name is Ken and he's always at events. He played Cohen in a fan movie of Troll Bridge. The lady in the leather barbarian gear is Demdike, who posts here now and again. The guy in the basque is Ptim who is one of the original members of this forum. The guy with the horned helmet is Butcher - I don't think he's a member, but his partner, Periwinkle is. Both of them were in The Colour of Magic movie.
There's a couple of others I recognise from other events as well - including Mr Boggis in the background - he was the model that Paul Kidby used (I believe) for his pictures of Mr Boggis of the Thieves' Guild.