This could either end up an unforgettable night, or something everyone just wants to forget so let's make sure your bases are well covered.
I'm a bit concerned that you're only 'sure' that your partner will be up for some group action. You need to be 100% on the same page about this and make your partner fully aware of any plans. If there's anything your partner may be uncomfortable about, then you have the chance to discuss it before the big night.
I am however encouraged about the fact that your friends are OK with sharing a room and understand that at the very least, if sex is happening, it is going to be in the same room. Of course, this doesn't mean they are going to want group sex. Your friend's partner may love to have sex, but maybe not with other people watching!
The best thing to do is let the girls get their feelers out (literally!). After a night of drinking, girls are bound to talk about naughty things so perhaps the female half of your party can work out what the other party wants to do sexually. If the two girls are bi and attracted to each other, that's an awesome ice breaker. The men folk should just watch and wait for an opportunity to join in. Trust me, there will be one.
If the girls are not bi, then the safest bet is to start off with same-room sex, which is what I would recommend to any couple new to swinging. Once you are hot and heavy into your own session, you can gauge the other couple's interest in joining in. Perhaps you could comment on how hot they look together and then ask if they want to join you both on your bed. If they are keen on the idea, they won't need to be asked twice. If they seem happy to stay on their own bed, then you have met the boundary of the night. Either way, I think you'll have a lot of fun.
I should add on a final note though, that if there is even a remote chance that your friendship will be affected by what you have in mind, then don't do it. If your friends have never considered swinging before, buyers remorse can easily set in the next day and you could find yourself minus two friends. This is another reason you may wish to just limit yourself to same-room sex at the most. There's always next time!
Remember, RedHotPie is full of couples who are very clearly into group sex. Why not remove the guesswork if your plans don't work out, and make some new sexy friends for another hotel session?