Add someone as a contact to your Teams so you would be able to communicate through Teams easily.
At the top of the Teams window you can find search input
Just type email address in the search bar and click on Search externally:
An you will gain requested external contact:
And now you can click on the contact and communicate:
Thus, you are using Teams to collaborate and chat with external contact. Fast and easy. Also, you can start a conversation from Outlook directly. Hover your mouse to the contact, wait for the context window and click to the message icon.