Alizizou

Nantra Silom is definitely NOT charging any guest fees, in fact they don't care at all whom you bring and when. 
 
You will be charged 200 baht if you request an additional towel which will be a case of threesome or if you bring another guest on the same day and you want him to have a fresh towel.
 
Firecat is right, chances of  having fee problem are minimal as long as you pay for double room .
 
Those hotels who charge such a fee should be publicly shamed here so we will not be supporting them.

sorry to say, they do. At least they did when I stayed there quite a long time ago. Not very friendly about it, too, I found..... I hope for you that this has changed.... please report

yes difficult to understand, at least with western logic. Might be because:
 
- the guest is not registered and therefore doesn't qualify as the second person the room was booked for. If you register your guest, there should be no fee, though of course then you are stuck with that registerd guest, and bringing others on other days will not work, or rather incur the "joiner fee"
 
- they want to discourage bringing "guests" to the rooms (for whatever reason, maybe because they don't want to be seen as supporting prostitution)
 
- make some extra money off of the sex tourist
 
- some inecplicable Thai logic