Most of the top end hotels don't seem to appreciate you bringing company home. They will look down their nose at you, give you pain and make it clear that they don't want your business.
Mid-range places like the Salathai resort have a sense of humour and will put up with you, though they do get a bit upset if you bring home a different girl every night.
The 'joiner fee' is a hot topic which everyone seems to have different experiences of. It seems that the best way of answering your question is to e-mail the hotel in question and ask them directly about their policy for 'overnight room guests'.
I have loads of these because I sat down one night and e-mailed every hotel I could find in Patong, an interesting exercise.
