Starting from this year, when my Malaysian friend with Thai driver's licence flew from BKK - udon thani and back, they won't accept her driver's licence anymore, they asked for passport.
Grab pick up is on the outer lane once you exit the arrival hall.
The inner lane is for airport limousine. Cross to the outer lane, that's where Grab or personal pick up is located. Grab pick up will list several exit. Ensure you select and wait by the correct exit door.