Are you meaning the very large groups like the Pickup Games, the GM Academy, Knights of the Written Word 2, and various living world/adventure paths? They are just regular groups that grew and expanded over time to be more of a specialized group. The members help each other with specific questions about systems, playstyles, character building, and other things that are considered off topic in the general community forums. You post to their LFG forum that you are interested in joining (if they have specific instructions, follow them) and you are usually invited in. Gaming group creation instructions can be found on this wiki page.
