iPhone group chat participant not listed

Hi,

I have a group chat with 2 other people who use iMessage. They are the only 2 people listed on the group chat. The other day, a fourth person (married to one of the other 2) sent an iMessage on the chat. I scrolled back and see no point at which we got a notification that the 4th person was added to the chat. None of us added them. Further, they aren’t listed as a participant. How does that happen?! And how do I now start a group chat that is actually only with the other two people? When I enter just their two names to start a chat it takes me into the existing chat that mysteriously includes the fourth person. Thanks!