They just rotate in and out and may do so at the left front position, or into the serving position, or into the position just after the server. A combination of these places may be used if there is more than one player to rotate in. Where the players will be rotating in should be discussed with the opposing coach before the game starts when the line-ups are exchanged. Teams take a few minutes to warm up with some drills, etc. ending with each team spreading out behind their respective service lines and practice serving. If they don t have one, a referee from the other team may be used.
Referees go over the boundaries and specifics of the gym with both teams and coaches.
Ref may also discuss how they will call the game with both coaches. Referees go over the lines and placement of line judges. Referee makes sure there is a gym or stage door monitor in place before any game can start. Both teams should share in this responsibility. No child is allowed to go into the hall or use the restroom without adult supervision.