You can't change the people in an existing chat. But, you can start a new message and add or remove people from the previous chat.
- Go to Google Chat or your Gmail account.
- On the left, under “Chat,” select a group conversation.
- At the top, next to the conversation name, click the Down arrow or More options Add people & bots.
- Enter names or email addresses of the people you want to add.
- Click Send.
- Optional: To remove people from the conversation, at the top, next to the conversation name, click the Down arrow or More options View members.
- Next to their name, click More options Remove from room .