The groom usually chooses a few men who he can trust for taking care that everything goes well on the D day. He usually chooses his close friends, colleagues and family members. Once he chooses who to pick, he divides the duties among them. This way, every person gets a specific task list and there is no chaos.
The best man has the most crucial role to play. He has the most responsibilities. He needs to see to it that everything goes well. He is also responsible for arranging the bachelor party. He is responsible for arranging for the travel of both the bride and the groom. He is also responsible for supervising the ushers and other grooms men. He takes care of the ring and the wedding license before the ceremony. He needs to see that everything goes well and the party is going on smoothly.
The other groomsmen and the ushers are responsible for the hospitability. They need to make sure that the guests are seated and they are comfortable. They need to unroll the aisle runner before the bride enters. They need to ensure nobody is left behind from the reception. They are also expected to introduce guests to each other.