Join forces

Join forces is an ability word introduced in Commander.

Join forces allows multiple players to pay mana to increase the effectiveness of the same card. Starting with the controller of the card with Join forces, each player may pay any amount of mana. Then, an effect based on the total amount of mana paid by all players occurs. All join forces cards included in the set have symmetric effects that benefit all players equally.

Rulings

 * The opportunity to pay mana happens when the join forces ability resolves. If the join forces ability is on a spell, and that spell is countered, none of that spell's effects will occur, so no player will have the opportunity to pay mana.
 * Each player in turn order gets one opportunity to pay any amount of mana. Once all players have had this opportunity, the join forces ability continues resolving.
 * The mana paid for the join forces ability of a spell doesn't change that spell's converted mana cost. The converted mana cost of Alliance of Arms, for example, is always 1.
 * None of the join forces abilities have targets. Each player may pay mana, and each player will benefit from the four sorceries with a join forces ability.
 * One creature (Mana-Charged Dragon) has a join forces triggered ability.
 * Spells with join forces abilities ignore the spell's mana cost when determining the total effect. For example, the spent to cast Alliance of Arms will not add 1 to the total mana paid for the join forces ability.
 * Spells with join forces abilities will have the same effect for each player based on the total amount of mana paid for those abilities. It doesn't matter how much mana any individual player paid.