Per discussion with Fremoc, the Clarification stands as follows:

Any Member of the DJB may participate, so long as at least one of the members participating is from a unit involved in the Feud.

Fremoc has also changed his mind on the Dueling. You need not Duel in Outlaws Den. A SCREENSHOT however, remains required. It must be a SCREENSHOT of the winner above the loser's corpse.