If you're using the actual guest passes, and not comp tickets, the CM who is doing this needs to be careful. Disney is starting to crack down on Maingate usage. Anytime a CM uses their Maingate passes to get guests in they are supposed to accompany them the whole time. Comp tickets, however, can be used without the CM being present.
So if the CM if going to get you in with the maingate passes and then leave, I would be very leery about letting you guys make FP+ selections at the kiosk day of. If they ask for the CM, and they aren't with you, it could be cause the CM to get in a lot of trouble. As always, some CM working at the kiosks will probably be lenient whereas others may not be.
The CM can set up fastpasses for the guest passes 7 days in advance though.