A paperback novel I read mid 1970's UK.
Set in a USA town during a long heatwave, a lot of good looking strangers (who it is eventually revealed are from a few hundred years into the future) are moving in and taking up all the motel rooms and rented accommodation.
Male protagonist begins to take notice and starts a relationship with one of the people, he takes her for an expensive fancy meal but her reaction is "it's only food".
One evening he's in a club with her and as she walks across the room a section of her dress turns transparent and her breast is on show for a minute or so. All eyes are on her after that!
She explains it's a perfectly decent dress with a built in factor to randomly do this and she can't understand the fuss.
His suspicions grow about them being time travellers and he confronts her and discusses a future where fertile land is in short supply, so a populace are conditioned to regard food and eating as just another exercise/chore. Also where fashion is very revealing.
She takes him to meet some of her party for updates of what will happen regarding the heatwave.
That's all I got, I can't remember if they turned out to be benign or bad.