OP.... As has already been indicated, members of the public meeting passengers at BKK are not allowed into the baggage reclaim area as this would mean passing through the customs control which is a restricted area.
When you exits from the customs control area (which is opposite door number 9 that leads to the roadway) turn right and make your way along the wide walk way towards door number 4 where there is a designated meeting point that is marked with a big blue and white sign suspended from the ceiling.
FYI... members of the public meeting arriving passengers are not permitted to enter the wide arrival corridor past door 4 - only official/registers drivers/guides are allowed to go to the exit from the customs control area.
Suggest you wear a red rose in your button hole or carry a copy of the New York Times so your friend can recognise you
Hope this helps and all goes well.