A best man for a wedding does not traditionally pay for a bachelor party. He arranges the party and keeps track of activities, guests and accommodations. He makes sure that each person attending pays a fair share of the cost.
The best man serves as the primary assistant to the groom at the wedding. He is equal to the maid of honor, ranking just after the bride and groom in importance for the wedding.
Additional responsibilities of the best man include overseeing the ushers, standing next to the groom during the wedding ceremony, keeping the rings until they are called for in the ceremony, acting as a witness to the marriage, signing the marriage certificate, assisting the groom as needed on the wedding day and giving a speech at the reception.