I imagine in negotiations a potential new entrant to any alliance would have to demonstrate the commercial value that would be gained to the existing members as well as itself in being included in the alliance. If you you have commercially successful pre existing relationships with the other carriers than the intergation would be a lot quicker and simpler - and the existing members would be more likely to vote you an invite.

I presume in OW, AA & BA ( and in *A SQ, LH & UA) would have a veto on certain issues - definately over new joiners.