What do we call a group of stage actors?

Solution 1:

If they come together just for a single performance, they are called the cast of the play. However, if they regularly perform together, they may be called a troupe or repertory company.

Troupe generally applies to groups that travel from one venue to another, and may sometimes be used for groups that tour for only one show. Repertory company refers specifically to a group that performs multiple successive shows together, and usually has a stationary home theatre (though repertory companies may also tour). From Oxford Dictionaries:

cast² NOUN
treated as singular or plural The actors taking part in a play, film, or other production.

troupe NOUN
A group of dancers, actors, or other entertainers who tour to different venues.

repertory company NOUN
A theatrical company that performs plays from its repertoire for regular, short periods of time, moving on from one play to another.