Attend wedding-related functions, such as the engagement party, bachelor party, and rehearsal dinner.
Purchase a gift for the couple (this can be a joint gift from all of the groomsmen, if you all wish to do that)
Helping, if necessary, with escorting guests to their seats, understanding any seating diagrams, etc.
Assisting in any last minute duties in setting up, or taking down, any ceremony decor, if necessary.
Helping to return the suits, if they are rented.
In addition to the groomsmen duties:
Plan the bachelor party.
Deliver the first toast at the wedding reception.
Acts as a witness for the marriage certificate
Coordinates the schedule with the groomsmen and ensure they are well-prepared for their duties
Holds the ring during the ceremony
Assists the groom in general duties, such as vendor-related deliveries and drop-offs.