The consumption meters used to bill Party are player-oriented: player network-minutes, player voice-minutes, network egress etc. Xbox Live players will not increment these meters and effectively be zero rated. PlayFab tracks this using the LoginWithXbox API so please use this API to enjoy the Xbox Live program.
Usage from other identity providers or custom IDs will be billed normally; check out PlayFab pricing and PlayFab Party pricing to see how standard Party activity is metered and zero-rating applied.