In multiplayer, when a player moves a unit to an open battlefield (no defender), can all other players join the showdown?

Ruling: When a unit moves to an open battlefield with no defender, all players are relevant and can play actions in response to the showdown. However, in a 1-on-1 combat with a defender and attacker, the showdown is only between those two players unless other players are invited to join.

Nuances:

• Open battlefield showdowns allow all players to participate
• Showdowns with a specific defender are limited to attacker and defender unless others are invited