Why can’t I add someone to the iMessage group chat I’m in?

I’ve added people to the group chat before but last night when I tried to add someone, it wouldn’t work. I tried restarting my phone and it didn’t fix the issue. I had my sister add the person and it let her do it. Then today, I wanted to add someone else. My sister’s busy so I asked a couple other people to do it and neither of them could do it either. Is something going on with Apple? Was there some recent change where only the person who made the group can add people even though the add contact option is still there for the rest of us? Does it not work for other people once you hit a certain number of people in the chat? To be clear, everyone currently in the chat is on iMessage and the people I’ve tried to ask are as well.