Generally, I don't think it's a problem when actors arrive early. I would rather have them there early because they left extra time for traffic, public transportation, carpooling or whatever reason than have them be late. If you ask them not to arrive early, you risk them arriving late.
If it's a matter of them not being able to get in because the stage door is locked, they could find a coffee shop or somewhere else nearby to hang out until SM gets there. If there are people there to let them in (as you mentioned), can they hang out in the green room or somewhere until the dressing rooms are open? To buy a few extra minutes, can you open the door at 6p when you arrive, instead of 6:15p?
In all of the contracts I've worked, I'm not aware of any Equity stipulations about when actors "can" arrive for call- it's something that's worked out by the theatre itself. (For instance, some theatres give out key fobs or something similar, so that actors can let themselves in.) If the theatre doesn't want actors arriving before stage management, I would remind the actors that the stage door will open at X time, and if they show up early, they are risking having to wait until stage management arrives, because that's when they will be able to get in.