I am having the same problem. An old number of my sisters got included in a group text. The person and his wife have gotten upset. I tried to delete them (I'm the group text originator), but couldn't find an option under details. I tried swiping there name. I tried to do it from my phone and my mac. All are running latest software.
I looked up on page 79 of the iPhone manual, gives details about group messages. Sounds good in the manual, but unfortunately, it doesn't work as instructed. I click details. I am unable to add anyone to the group or delete them. I cannot withdraw from the group. Actually, a couple of my family members wanted to withdraw (they own iPhones also), but were unable to do so. The only option any of us had under "details" was "do not disturb," which mutes conversations.
That's it.
What am I missing here? We have no control over group messages once started? We can't add to them or delete or withdraw from them?
I sure hope there's an easy fix for this beside just a mute notifications option.