In the league I help run in GTR2 we generally rig the grid by all going out on track together, forming into grid order on the start and all doing a lap in that order. We call out our time as we cross the line and the guy behind goes slower. It requires everyone to be on TeamSpeak though, which is as good as mandatory in our league.
Another way is to connect in grid order but it requires everyone on TS also
Granted we have the server manager program for GTR2 which allows is to use the !swap command to do it that way too. The problem with that is that by the time you've swapped a whole grid around someone will get bored in the pits and end up Dq'd for pushing cars around in other garages. Or someone will get DC'd and have to rejoin.
It can be a bit of a pain and I'm thinking of proposing a few changes for next year that will make everyone qualify prior to the race as it's so much easier that way.