Groomsmen: A groomsmen is someone who is important to the groom, and whom the groom wants to be a visible part of his wedding. He may be a close friend, brother, cousin or father. But a groomsmen does more than stand around and look pretty. He also serves as advisor to the groom, beforehand, helps the groom be organized, aids with any wedding planning details he can, and helps seat guests at the wedding if there are no ushers.
Ushers: Ushers serve as the official greeters of all guests at the ceremony and as such should be in place one hour before the ceremony begins. Very often a head usher is appointed to oversee the ushers, which relieves the best man of both having to attend to the groom and keep watch over the ushers.
Many weddings use the groomsmen as ushers as well.