Those members of a theatrical company who appear on stage at the beginning of the first act. The preparatory warning 'Overture and Beginners' is a signal for the ochestra to start the introductory music and the 'beginners' to get into their opening positions.
A call given by the DSM to bring those actors who appear in the first part of a play to the stage. e.g. "Act One Beginners to the stage, please". The actors/actresses are then called by name.
The backstage call 5 minutes prior to curtain up to get the cast into position for the first scene.