Traditionally the groomsmen represented bodyguards for the groom, to protect him during the time leading up to his wedding, ensuring that he marry his bride successfully. Today, groomsmen are more symbolic but this does not mean that they do not have any duties.
Like the rest of the wedding party, the groomsmen are usually members of the family or very close and trusted friends but who are old enough to follow an itinerary, hold some responsibility and with whom the groom feels capable and fit to bestow this honour.
Sometimes the groomsman may receive a gift for his services. If chosen as a groomsman, be prepared that you will have to set aside a budget for monetary contributions towards your duties.