Add People to a Chat in Teams
Add People to a Chat in Microsoft Teams
To add more people into a Chat, select Add people 
in the top right corner of Microsoft Teams. Then, type the names of the people you'd like to chat with, choose how much of the chat history to include, and select Add.
Note: When adding someone to a group chat (not a one-on-one), you can include the chat history.
Microsoft Teams saves the entire chat history, all the way back to the first message. If someone leaves the Chat, their Chat responses remain in your Chat history.
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