Mostly it is a mix of bathing suit and "pool" type wear from modest to sexy. Your results may vary especially if it is in someone's back yard that has no neighbors and a tall privacy fence, no privacy, at a hotel pool in the open, or even if someone "rented" out the neighborhood pool. There are all actual situations called a "pool party" we have attended or choose not to attend due to the lack of privacy (the rented neighborhood pool). I recommend both talking to the host and finding out the level of privacy you should expect and for your first one dressing as modest as you would at a public each or pool. It is easier to take things off as you get comfortable than cover up if you have nothing modest to wear. It also helps you not be "the first" or even "only" one dressed sexy or naked perhaps having the effect of seeming experienced when you are not. As your comfort level goes up you can relax your modesty if you want or not. As far as what to expect it is a swinger's party so expect that sometime during the event you will be exposed to it. This is especially true with a pool party with privacy because wet mostly naked (or naked) bodies tend to lead to such things. In the next paragraph I will describe what we normally encounter and wear at our "club's" pool parties.
The pool party we regularly attend offers a great deal of outdoor privacy. Mostly it is a mix of bathing suit and "pool" type wear from modest to sexy. That said it really varies from nice dresses to naked (personal preference some never get in the pool). There has also been blowjobs and even sex by the pool (sex is not allowed in the pool or hot tub). My wife started out somewhat modest in a triangle top bikini with full coverage bottoms and now simply wears a thong bikini bottom and cover up. The cover up is more for warmth after getting out the pool than modesty. She has gone completely nude but prefers the bottoms for an air of "mystery" Me I typically wear a pair of swim shorts and a summer quick dry button up shirt outside the pool just the shorts in it. We do switch into a dress (her) and shorts and shirt (me) when we are done swimming. Typically though swim time ends when playtime begins for us so the "outfits" sometimes never get worn until leaving LOL. I do need to add at every pool party we have ever been to that allows nudity there is always at the very least this one guy and woman (together or not) that would walk naked from where they parked if they could. The funny part is for some their partner is usually wearing something a lot more modest while they are letting it all hang out.
