Players on the serving team can take any legal position on the court they want, but they cannot jump around, wave their arms, or move laterally during the execution of the service.
If a player on the receiving team can't see the server, it's his/her responsibility to move.
A screening call would only be whistled a fault if the trajectory of the ball's flight passes from the server, over the player/players and to the other side. So, if you have 2-3 guys bunched at the net near position III, but the service goes straight down the sideline, a screening fault would not be whistled, because the ball didn't travel over the people at the net.
In practice, since neither the 1st or 2nd ref have a good enough angle to see the trajectory of the ball - especially at the higher levels of competition - screening is rarely whistled.
It's a rule without teeth.