The primary role of the groomsman is to assist the groom, in every way possible before, during, and after the wedding. Groomsmen are usually family members or close friends, and are chosen because the groom holds high regards for them.
In ancient times, the groomsmen were actually guards of the groom, and their primary responsibility was to protect the groom, and ensure nobody prevents him from taking his rightful place, beside the bride on the ordained wedding day.
If these duties are expected from our groomsmen today, I'm sure no guy would be ready to become one! Thankfully, the responsibilities called for today are far simpler.
Before the Wedding
Prior to the wedding, it is the duty of the groomsman to help the groom with different wedding arrangements. He also assists with tuxedo shopping, alteration meetings, and even helps the best man organize the stag or bachelor party. Moreover, it's the groomsman's responsibility to get the couple's wedding car decorated before the wedding. He is also the one who helps the groom get dressed for the wedding, and ensures the groom keeps a cool head on the wedding day. Running around for every last minute hiccup is also part of the job. If there are two or more groomsmen, the duties will be divided among them.
During the Wedding
At the wedding, it's the groomsman's responsibility to greet the guests, and have them seated in the pews. With this comes the responsibility of ushering the guests as well. In Christian ceremonies, the bride's side sits on the right, and the groom's side is seated on the left. It's the groomsman's duty to ensure people find their side. Moreover, during the wedding ceremony, the groomsman has to stand next to the groom, and cater to any immediate need that arises.
After the Wedding
Gift management is also part of the groomsman's responsibility. He should know how to collect the gifts being given, and must know where to store them safely. He must also have a plan ready as to how to pack the gifts quickly, and have them delivered to the required destination after the reception. Last but not the least, the groomsmen are expected to entertain the bridesmaids at the wedding reception, dance with them, etc.
If you are asked to be a groomsman, make sure you attend the rehearsal, so you know what you are supposed to be doing on the wedding day. Your responsibilities and duties should be clear in your head, because you don't want to screw things up. Clarify all doubts before the wedding and stay focused. In a nutshell, as the groomsman, you are to make sure everything goes on smoothly before, during, and after the wedding.