As with all factions, there are those who are directly affiliated to it. Then there are those who are drifters, who come and go and then they're those who are what they are by definition, but not directly affiliated to the faction. Sith such as Kaine Zambrano, who has always been involved in the Empire, to Mikhail Shorn who flits in and out whenever he feels and then Darth Moridin, who is a Dark Lord of the Sith but not involved in the Empire anymore.
There can be this Dark Jedi faction, where several Dark Jedi have gathered to work together and then there can be Dark Jedi such as Domino, a vigo in the Black Sun Syndicate who isn't necessarily (not at this time anyway) affiliated to a Dark Jedi faction, as it were.
Cliche as it sounds, we called our faction the "Dark Order" in my other community. It's nice, simple and to the point.