I got a pop-up for my sim who wanted to go on a date with his wife. I accepted and went through the date UI as expected, but did not choose any additional sims to invite (or "guests"), just him and his wife. I got to the venue and their teen had joined them. I was not able to send her home, because she was considered part of the event, even though I had not selected her.
I will note that the teen was my currently active sim when the pop-up occurred, so maybe the fact she was the active sim somehow forced her to be included. I will try to see if I can get it to happen again, but it certainly seemed like a bug.
(I'm sorry if this has already been reported. I searched and couldn't find this specific issue.)
