Microsoft - Add someone to a call in Teams
This article will show you how to add a person to a live call.
Add someone to a Teams Call:
To add someone new to a group call, click Add people 
at the top right corner of your call window, then type their name in the box at the top of your call roster.
As of right now, you can't add anyone new to a one-on-one call. But you can make it a group chat, convert it to a call, and then add someone.
Quick Video:
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