There is some amount of face to be gained by a Thai inviting you to have a drink with their table full of people. There is also some amount that will be lost if you refuse.
I’ve refused, but when I do so I make it seem that I’m really in a hurry to get somewhere or that I don’t drink and drive… or something else. I usually hurry to get away if I don’t want to drink any Lao Kow (rice whisky) or whatever else they’re pouring.
It’s best just to accept, drink one and thank them profusely, smile a lot and then leave as soon as possible. If you stay one of a couple things can happen.
1. You get drunk quick, and maybe that’s your thing and you enjoy the experience. You buy some drinks, they buy some drinks – it’s a mutually symbiotic relationship.
2. Their real reason for inviting you for 1 drink is so you buy the next bottle of Johnny Walker Black. It’s a parasitic relationship. If you refuse to pay for it – they’ll take it as an insult and beat you. (Happened to a long-term expat friend of mine at a bar on Soi Bangla)
I have yet to stay more than 1 drink with any Thais I don’t know that invite me to their table. I’ve refused about 10 times. I’ve accepted about 10 times. I’ve never paid for the next bottle.