Maybe I'll quickly put in a Dark Council post here.
The Dark Brotherhood does not have any policy in place that prevents dual membership with any other club. This is also not foreseen to change except in the unlikely case that a club would directly attack the Brotherhood.
However, since a dual membership needs to be approved by both involved clubs to work, you should know that the Emperor's Hammer and all its subgroups prohibit dual membership with the DJB. If they catch you being a dual member, you'll be expelled from the EH. To my knowledge, no other club has any such restriction.
The Dark Brotherhood also has relations with multiple other Star Wars club. Formal alliances exist with the Rebel Squadrons, the Star Vipers, the New Republic 5th Fleet, the Galactic Empire, the Force Elite Soldiers, the Imperial Directorate and the Galactic Imperial Order. Within a full alliance, it is generally acceptable to use Brotherhood resources to perform some limited recruiting for dual membership. (This does NOT mean it's acceptable to spam members with unwanted email, but you can do things like short messageboard posts of a "The XYZ club is looking for more members enjoying activity ABC", especially if said ABC is an activity not currently supported by the DJB.)
"Semi-alliances" exist with a few more clubs by means of the Interclub Training Event (ICTE). The Imperial Dominion (I wouldn't know a club called "Imperial Domain", so I assume you mean the recently formed Imperial Dominion) is an ICTE participant. During the ICTE, you can play games with members of all allies and ICTE participants and gain Brotherhood reward (or reward in another club of your choosing should you rather choose to represent them) for them.
Membership in other clubs does not affect your status in the DJB in either direction. If you founded a club and it grows to epic proportions and you then join the DJB, you're a normal Initiate while you're acting as a DB member. (You'll of course still be afforded the respect due a hopefully allied commanding officer when acting as such
). As a logical extension of this, you'll want to join only as many clubs as you want to be active in. Since you're splitting activity, you'll progress slower in each individual club and it's not that fun to still be in the bottom 3 ranks everywhere after a year of play
And, should you wish to organize a joint activity between two (or more than two) clubs, be sure to follow protocol. This means, get permission from the command of the originating club first, then contact leaders of the allies you wish involved who are your own equivalent in status. A DJB Quaestor shouldn't go to the RS Fleet Commander for having a competition of his House vs. one of their comparable units. They go to that unit's leader and it'll be that leader's responsibility to then get permission inside their own club. Going directly to the commanding officer is something you should only do when acting as command staff or with direct authority of your CO. Organizing interclub activity can be challenging and sometimes frustrating as you'll be coordinating different cultures but if you can pull it off, satisfaction and reward can be great.
I should also mention interclub activity that WILL LIKELY get you into trouble:
- Any and all recruiting that is aimed at getting a DJB member to leave the DJB in favor of a different club.
- Overly aggressive recruiting especially in the form of personal messages or unwanted e-mails. Do not personally target people unless they have explicitly shown interest first.
- Using DB facilities for recruiting to non-allied organizations competing for the same activities. (This means if you want to advertize to your Star Wars CCG group, that's okay because the DB doesn't do the CCG, but recruiting to your Jedi Academy group would not be looked upon favorably).
- Advertizing any group that promotes illegal activity, condones adult content or is otherwise not suitable for membership of all ages 13 and over.
(The preceding is subject to revision by the JST, I just wrote down what is going practice around this place).
So, bottom line:
- Enjoy dual (triple, quad...) membership
- Be active in both clubs
- Do not annoy anyone with recruiting
(Quite long, but maybe we can turn this into an official primer of "Interclubbing for DJB members" down the road).