Hey there, Pravit
The simplest way to link multiple commands together is to throw an ; between them.
So in this case:
?((roadguard knowsabout aP)>0): "_x assignascargo car"; foreach units roadguard; leader group roadguard assignasdriver car; "[_x] ordergetin true" foreach units roadguard; roadguard doFollow aP
It gets a bit clunky with multiple commands like that (especially in a trigger).
Another alternative is to write a dedicated script to execute those commands, but personally I prefer to use a script only if really necessary
Anyway, hope that helps you out