Rule 316.5.b.1
In Modes of Play with teammates, the Turn Player's teammates may play spells and activate abilities, including ones without Action or Reaction. They can only do so when the Turn Player invites them to do so with their own Priority.
Loading Arbiter…
In Modes of Play with teammates, the Turn Player's teammates may play spells and activate abilities, including ones without Action or Reaction. They can only do so when the Turn Player invites them to do so with their own Priority.