How does the 'invite' mechanic work in 3-player Riftbound games?

Ruling: During a showdown on a controlled battlefield, only the attacker and defender are Relevant Players by default. Either can invite a non-Relevant Player to join, and if they accept, they must immediately make a legal game action and remain a Relevant Player for the rest of that showdown. Outside of showdowns, all players are always Relevant Players and can play into any chain without being invited.

Sequence:

• Attacker or defender (whoever has priority) invites a non-Relevant Player
• The invited player accepts or declines
• If they accept, they must immediately make a legal game action (e.g. play a reaction)
• They remain a Relevant Player for the rest of that showdown

Nuances:

• Inviting can only be done when you have priority during combat
• Inviting someone doesn't imply any kind of allyship
• On uncontrolled battlefields, all players are automatically Relevant Players
• Outside of showdowns, all players are Relevant Players by default and can participate in chains freely