How does the server verify that all players have loaded?

[Djavu 9]

Hello everyone, I need help. How to check if all players have already loaded their characters and only then start the mission? What I intend to do:
The players respawn the mark respawn_west, where they have a trigger that will teleport them to a helicopter, which will be in a position and will take them to the base. But the helicopter will only start the move when the P1 units are shipped. A black screen with some information will appear for those who have already been charged and teleported while waiting for the rest of the players.