Some of my best Porshe friends own GT-3's and I would never hold it against them or look down on them just because they look like dentists who hugely overpaid for a car with the engine in the wrong place.
Ok, on a more serious note, Boxster's fit right in just fine into the PCA scene. Don't give it a second thought.
Just remember that like most clubs, you get out of it what you put into it. I'd say jump in and get involved in a part of the club that you're interested in. Its easy to make some new friends who share the same passion.