They're just groups that members can join.

The "No Postcount" group is a group of members who don't want their postcount to be shown.

The Dragoons are all Halo nuts, so unless you're a Halo nut and play it religiously on XBL, then you don't want to join.

PX-978-L3ARM is Chruser's group for something or other. Haven't a clue what, though.