My wife still maintains a close relationship with one of her high school classmates and his wife. She has taken several seperate vacations to go see them, and spend several days to a week with them. They were all in their early 50's by this time.
The very first time on one of these vacations, she kept insisting that she was too shy to get in their hot tub naked with them, although his wife would be naked and he would only wear athletic-type shorts. She would call me every evening to tell me what they had been doing, and I kept telling her just to get naked; there wasn't any reason to be shy or embarrassed. They had dated and fucked while in and after high school, so I had no idea why she would be apprehensive.
When she went back the following year, she bought a 1-piece suit to wear (so she siad), but I really believe it was just a "cover story". They had began some light three-way sex from the year before, and she was anxious to go there again to play some more. She bought the suit to 'convince' me she would wear it in their tub, but I think she was comfortable being naked with them by this time.
For the third year, she didn't take that suit--or anything else--to specifically wear in the hot tub. She did buy a few pairs of thong panties to take and wear up there, so I insisted that she wear them under a plain white tshirt in the tub. That way, she could still feel like she was 'wearing' something, but the guy would still be able to totally see her big tits and nice, big ass. It turned out that was the night the two girls had their first lesbian oral sex, after my wife had been fingering the other wife's pussy and sucking her tits. And as my wife began licking the other's pussy, the guy came up behind her and fucked her "from behind" (doggy).
There are some pictures and possibly some video that exist, but they've never shared them.