The main players in an encounter are the offense, the player currently taking a turn, and the defense, the player with whom the offense is having an encounter (which is generally the player whose color was drawn from the destiny deck). Other players are not involved in an encounter by default, but may become involved if they are invited by one or both main players to become allies.
Most alien powers (31 out of 50 in the FFG base set) can only be used as a main player.