Even if you book a room for 2 persons, you may be subject to a "joiners fee" (usually from 800 THB to 1500 THB) when taking one of the local lads to your room. Joiners fee is not customary in BKK, but predominant in Pattaya and particularly Phuket.

You may try to sneak your guest into the room, but if the security guards or front desk staff notice it, you will have to do the walk of shame to the reception to sign/pay.