It's a quest. Rescue the scientists from the purifier and escort them to the citadel. Talk to some Brotherhood guys and they will tell you that you have to talk to Elder Lyons. He then gives you permission to be trained by Paladin Gunney. There is the Brotherhood Outcast members (i just kill them to get thier cool stuff) and the enclave but I don't think they give you the training at all to wear it.