A troupe system is a way of playing role-playing games in which a group of players takes different roles at different times.
[1][2] In Ars Magica, the troupe's characters all belong to a single Covenant - a small group dedicated to the study and utilization of the magical arts.
Each player's primary character is their Magus, one of the wizards who form the core of the Covenant.
Each player also creates a Companion, a "skilled person who aids magi but is not necessarily in their pay."
Finally the group (or troupe) as a whole creates a number of Grogs, mercenaries and other servants hired by the magi.