There is no notion of ownership of a group chat and any participant may add another App User to the group chat at any time. The added App User will only be aware of the conversation post being added, the existing history will not be made available to them. Other participants in the group chat will be informed of the new participant entering the group chat.
Group chats may have many participants and these participants may add new participants or leave the group chat as the group chat progresses. In this way the participants in a group chat can change organically as the chat progresses.