There's always someone that may not be the early bird of the group, and then there's people that are. Know your bridal party and schedule accordingly. It doesn't matter who goes when as long as someone's in the chair.
Keeping a schedule never has to be exact of the order, it's more important that the time blocks stay on schedule.
Don't feel badly about asking people to come early, it's part of the gig. We always make it fun with music and laughs.