A mixed group is any kind of cooperative gathering of players who play characters part of factions which are naturally opposed to each other. For example, a ranger teaming up with a group of hunters. This is made possible by the unrestrictive nature of the faction system – players are free to play with whoever they want and forge their own alliance and loyalties without artifical restriction.
The in-game group system and the legion system can be used to facilitate teaming up between players of opposing factions, but is in no way necessary provided the parties can safely identify each other and communicate. Third party voice chat software such as TeamSpeak or Skype are often used to achieve coordination prior to forming the actual "mixed group" using Breaking Point's group system.
The existence of mixed groups can sometimes make approaching an unknown player of any allied faction just as dangerous as encountering an enemy faction player. Players are advised to communicate with each other to avoid being ambushed by enemies which may be using mixed groups to their advantage – a high level ranger, nomad or survivalist using the faction chat channels to lure their allies to danger, or simply listening in and revealing their position, in order to have them killed by players of an opposing faction is a common example of this.
Additionally, players of opposing faction may team up with their friends who have become traitors to their own, allowing both parties to progress and protect each other from both sides.