Here's a Console command:
findclientbyname(NameHere).setcontrolobject(objectidhere);
Sets a player's control to the object of your choosing, like a vehicle.
Walk up to a vehicle, type "/Getid" (No quotes)
Then, in the box that says (Objectidhere), place the id there.
BTW, the host can use this command to control players! Here's what my console command looks like when i control a player:
findclientbyname(evar).setcontrolobject(1256734);
(The 1256730 is my friend MP, lol i was controling him, it was funny.)
This topic is definently worth a sticky!