The term oneworld uses is sponsoring - the nomination/invitation could have been suggested by any carrier really - not that oneworld as an organisation probably doesn't already have a shortlist of potential members - but once a carrier signs up to join, an existing member will sponsor or co-sponsor the inductee. Sponsoring means assisting the nominee to get all their necessary system in line, the interline agreements in place, help train the employees, etc.
BA and AA have done most of the sponsoring, yes, but I believe IB was involved with the MX induction and CX with the JL setup, AY with MA and so on.
Who knows, perhaps LA will be the sponsor of the future mystery Brazilian member and CX would be the obvious sponsor of the Chinese carrier if and when oneworld can find one...