Cross-play means playing with and against players from different platforms. Cross-party means intentionally teaming up with players and joining a party from different platforms.
Killer Queen Black is now both cross-play and cross-party.
Killer Queen Black features an internal friend system, based on your LiquidID.
On Nintendo Switch:
Mixed teams constructed partially of local players, and online remote players is allowed.
When a primary player is invited to a party, all of their secondary players will be included in the party as well.
See voice chat for details of in-game and out-of-game voice comms.