I've been the plaything of a married couple
My best friend doesn't know that I've been sleeping with her brother. Her married brother, I should add. You'd think it's an affair, but his wife is in on the matter, it's just this kinky threesome thing we've got going on. I kinda feel bad not telling her, since we share pretty much anything else, but I feel like she'd be weirded out that I've been fucked regularly by her brother after he got married. Or that I've been down on his wife while he was fucking me.
The other weird part of it is that I have this big, not-terribly-secret crush on her brother's best friend, and I have a fair inkling that he might like me back but I'm not sure. Yet it hasn't stopped me from having sex with my best friend's brother. The sex is amazing though and I know I wouldn't stop even if I started dating his friend. Totally wouldn't mind getting him involved though, I'd love to have both of them bang me at the same time.
My situation is similar. I play with my friend and her boyfriend semi regularly. The person we are keeping it from is my older sister. My sister isnt actual friends with this couple, more like good acquaintances. We rather my sister not know just because she's the one that introduced me to them. My sister is kind of a goody goody and she would lecture me and possibly gossip to other people we all know so its just better if she doesn't know.
Another thing that is kind of similar is that my friend brought up the idea of having one of the guys she hooks up with over so it would be 4 of us. Im not sure if it will happen because of certain things, but I do like the idea. My friend has been introducing me to all kinds of fun things. So, Shhhhhhh! Don't tell my sister! Lol
As long as the four that are doing all the shagging, mouthing, sexing etc. or potentially may are happy to share and enjoy the group thing I think that is all that is important don't let any others know about it