The kicker here is that you cannot remove a group member unless you added them in the first place. The group member who added that member to the group is the only one who can remove them. They can also remove themselves too.
If you can't add or remove someone in a group message
- You can only remove someone if the group has four or more people, and everyone is using an Apple device, like an iPhone or iPad.
- You can only add someone to a group message if they're using an Apple device. If you want to add someone to a group message but they're using a non-Apple device, you need to create a new group message. That message will use SMS/MMS by default since someone is using a non-Apple device.
- You can only add someone to a group message that already includes three or more people. You can't add someone to a messages conversation that you're already having with just one other person.