I will try and explain how to call the script that Captain Winters posted.
; Script Call:
;
; 1: [15, 25, 2, _car1, _car2 ] exec "convoy.sqs"
; 2: [15, 25, N, _car1, ......, _carN ] exec "convoy.sqs"
Above is two examples on how to call the script. In both of them the first two numbers mean this; the first number is the minumum distance between units in the convoy, the second number is maximum distance between units. The next number is the number of units in the convoy.
; _stdis = minimum distance between cars--Minimum distance
; _limdis = maximum distance between cars--Max distance
; _nrCars = Number Of Cars in the convoy --Number of units
To call the script create a trigger. First name a unit, for the example I will use aP for the player and H for the vehicle I want aP to ride in. Now in the condition feild put aP in H -- what this does is check to see if aP is in vehicle H. Now in the onactivation feild you call the convoy script. [15, 25, 3, _car1, _car2, _car3 ] exec "convoy.sqs" -- In this example the script is for only three vehicles "_car1,_car2 and _car3". Do not forget to place a copy of the script in the mission folder. Try this out. The _car1 does not have to be _car1 it can be any name of the vehicle you want.