1) Ally oneself with 2) Collaborate 3) Go cahoots 4) Team up 5) Unite 6) Unite to fight 
Join forces is an ability word geared toward multiplayer variants. An effect denoted with join forces allows all players to contribute to it, usually by paying mana, though that effect might not be mutually beneficial. For example, Minds Aglow has each player draw a number of cards equal to the total amount paid by all players (so each player will ...