Group Chats
I started a group chat; they are not all iPhone users. How can I delete people or add people to that chat?
iPhone 12
You can't sorry. Since the chat is being sent via your cellular provider either SMS/MMS, once set, the group can't be modified. This is not an Apple or iPhone issue. This is an issue with your cellular provider. If everyone in the group was iOS, meaning the message was exclusively iMessage, you can add or remove people as that is entirely under the control of Apple.