From the Comprehensive Rules (Ixalan (September 29, 2017))
- 808. Team vs. Team Variant
- 808.1. Team vs. Team games are played with two or more teams. Each team may have any number of players on it.
- 808.2. Each team sits together on one side of the table. Each team decides the order in which its players sit.
- 808.3. Any multiplayer options used are determined before play begins. The Team vs. Team variant uses the following default options.
- 808.3a The attack multiple players option is used (see rule 802).
- 808.3b The deploy creatures options and limited range of influence options usually aren’t used in the Team vs. Team variant.
- 808.4. To determine which player goes first, randomly choose a team. If that team has an odd number of players, the player in its center seat goes first. If that team has an even number of players, the player to the left of its midpoint goes first. Turn order goes to the players’ left.
- 808.5. In the Team vs. Team variant, a team’s resources (cards in hand, mana, and so on) are not shared. Teammates may review each other’s hands and discuss strategies at any time. Teammates can’t manipulate each other’s cards or permanents.