Chick watching and intervening in sex between others

[spurious]

This might be a bit obscure, but I really like scenes where two or more people are having sex and there is a chick there with them who's not really involved but is watching, encouraging, commentating and occasionally reaching in a hand to help out (moving hair, guiding a cock, slapping etc). She's not really involved, but seems like she's there because she finds it hot. It's best when she is at least partly clothed.

Here is an example:

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I've seen several like this but can't find them. Any suggestions would be awesome!