Well, I suppose the first step would be determining which "member" of said plurality you intend to form the relationship with. Then getting permission from the other members of said plurality. While I lack much experience with plural individuals, in the case where there is a more "dominant" personality, one who is in the driver's seat more than the others, I would wager a guess that they would be the one most "stable" to be in a relationship with, given they will be present more often than any others.