Online multiplayer league event not allowing players to take over a car if late.

I have seen on some leagues that if someone joins late during practice or even during quali session (for as long as they had an invite to the lobby), then go in as Spectator and take over a car on the grid.
I now manage a league, but my drivers can not join the sessions, even if these are prior to the race. How do I set up the lobby for this to happen?