Whenever a creature token leaves the battlefield, put a +1/+1 counter on Twilight Drover.
{2}{W}, Remove a +1/+1 counter from Twilight Drover: Create two 1/1 white Spirit creature tokens with flying.
If Spirit tokens created by Twilight Drover leave the battlefield, they will trigger its first ability.
If a player leaves a multiplayer game, all creature tokens that player owns also leave the game. Twilight Drover's ability will trigger once per token.
Note that Twilight Drover doesn't have any way to sacrifice tokens or otherwise cause them to leave the battlefield.
The first ability only cares that creature tokens leave the battlefield. It doesn't matter where they went.