I'm a little behind on this thread, but we've had a lot of experience with our hot tub and swinging (the main reason we got it was for parties).
A few pointers...
I would not put too much emphasis on "portability", not if you want a spa big enough to party in. The most you can reasonably expect is "portable" in the sense that 4-5 strong backs and a heavy-duty 4-wheel dolly or two will be needed to move it.
Ours is 8' x 8' (holds about 500 gallons), which is about the minimum size hot tub you want for party purposes. Forget the "loungers". They are more trouble than they are worth. What you want is a seating arrangement where basically there is bench (and corner) seating around the tub's outer wall, and the center is sunk in for legs and feet.
If you have the bucks to where you can swing a Hot Springs spa or similar, go for it. Don't skimp up front, as cheap construction and components will come back to bite you. Make sure it has an "ozonator". This helps in keeping the water clean.
The only chemicals you really need are a non-chlorine "spa shock" (use weekly), bromide tablets and the floating container to hold them (it's only in the water when the spa is not in use). Defoamer (use as directed). Extra water filter and water filter cleaner (comes in a spray bottle). "Scum balls"; little foam balls you leave in the water with the bromide float, which collects any floating oils and the like from the water. You can get all this stuff at Leslie's Pool Supply or similar.
Submersible pump. You will want to replace the water every 2-3 months, and the pump speeds up the process considerably.
Temperature; realistically, if you are going to spend more than 15 minutes in the tub at a time, you do NOT want the temp cranked up to 105-110 degrees. Even in wintertime, if your intention is to spend an extended amount of time playing slap-and-tickle in your spa, set the temp no higher than 98-99 degrees. The water will be plenty warm enough for party time, but not so high as to do wierd things to your blood pressure and whatnot (that's why these spas all have warnings plastered all over them saying don't spend too much time in them, especially at "normal" spa temps).
SEX! Oh yes. You can have sex in the spa, although a certain acrobatic ability doesn't hurt. Of course, it won't be long before the issue of lubrication comes up, and there is a solution. Get some WET Platinum (it's latex friendly). It has a silicon base that allows it to go a LONG way in the arena of submarine fornication...
-- Bear