It seems like there are a few distinct groups at Threadless that occasionally do large-scale group submissions. Here are the ones I know of:

The Barons: includes both alumni and non-alumni. Fairly recently conceived, as I think they've only been a group for a few months. Past group themes have included Spears and Sci-Fi Nursery Rhymes.

Black Rock / Legion of Doom: I think this is two nicknames for the same group? Maybe? Legion of Doom has done some cool mash-ups (including a Design By Humans winner) and I think Black Rock is the name of the website they meet at.

Alumni: Official group for anyone who has been printed at Threadless. They have their own blog section, which has been used to organize the recent flood of (excellent) Purple Rain theme submissions.

What other groups are there at Threadless?