Klastri, the players can do what the NFL rules allow.  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.

Labels: moderation: safe