Some hotels in Thailand demand a "joiner fee" also knows as a "guest fee"; you have to pay extra if you bring a guest to your room. Typically, it is the family-friendly hotels and hotels on the expensive side that demand a joiner fee. The extra cost can easily exceed the amount you pay your guest.
It is understandable if a family-friendly hotel or an expensive hotel do not want certain kind of guests; however, instead of prohibiting that kind of guests, it is perfectly OK to bring them, if only you pay extra. This, of course, makes them hypocrites.
If you plan to bring guests to your room on your holiday in Thailand and if you want to avoid paying a joiner fee, you should send an email to the hotel and ask about their policies regarding a guest before you book the room. Remember to save their answer in case they change their mind later.
Alternatively, you can search the web for comments from other guests. Hotels that do not demand a joiner fee is also known as "guest friendly". Please be aware that if you search the web instead of asking the hotel, you may find outdated information.
If the hotel does demand extra payment for guests, you can consider choosing another hotel. Usually, most of the hotels in the "entertainment" areas do not demand a joiner fee; nonetheless, ask them if you want to be sure before booking.