On the sims medieval how does a bard recruit an actor?

Answer 1

Click the stage, select the play. Then an option will appear to recruit actor for anyone who is in the building.

Answer 2

Pick the Sim that you want as an actor, interact with him a little (it wont work unless you're somewhat friendly with the sim) and after a few "friendly" interactions the option to recruit him will be available, after that just click the stage and start your play =)