The NFL rule book specifically requires both teams appear on the field for the playing of the anthem, standing, remaining quiet, and holding their helmets in their left hands. Failure to do so can result in fines, suspensions, and the loss of draft picks. The rules are found on pages A62-63 of the league’s game operations manual:
The National Anthem must be played prior to every NFL game, and all players must be on the sideline for the National Anthem.
During the National Anthem, players on the field and bench area should stand at attention, face the flag, hold helmets in their left hand, and refrain from talking. The home team should ensure that the American flag is in good condition. It should be pointed out to players and coaches that we continue to be judged by the public in this area of respect for the flag and our country. Failure to be on the field by the start of the National Anthem may result in discipline, such as fines, suspensions, and/or the forfeiture of draft choice(s) for violations of the above, including first offenses.