"PLEASE ACCEPT MY RESIGNATION. I DON'T WANT TO BELONG TO ANY CLUB THAT WILL ACCEPT PEOPLE LIKE ME AS A MEMBER" - Groucho Marx in a telegram to the Beverly Hills Friars Club
First, I'm not wedded to the name 'the Inverse Groucho', but I've yet to come up with anything better, so do feel free to suggest something.
The Inverse Groucho is the realization and accompanying moment of uncomfortable clarity that to any outside observer you are in fact a member of a group to which you may not wish to belong.
I've had the moments in a variety of settings, though probably most often at metal and punk shows. Looking around at all the goofballs, then realizing I'm wearing the same black jeans, converse and plaid shirt as everyone else. "No!" says the voice inside my head, "You're different!". Untrue says my more rational, humble self.
Worse is that same feeling but in nerdier settings. Learning how to play Magic: The Gathering, seeing a comic book movie, or just playing World of Warcraft.
Experiencing an Inverse Groucho requires one to participate in activity without necessarily wanting to be a part of the community that surrounds those activities. So perhaps it's a bit of an anti-social a feeling, or the result of waspy, Calvanistic upbringings where having fun is always a little shameful.
Anyway, there ya go. The Inverse Groucho.