If you only want to have a videochat with them, then schedule a meeting. They don't need a Teams acocunt, they can join the meeting anonymously using the web application in a web browser.

https://support.microsoft.com/en-us/office/join-a-meeting-without-a-teams-account-c6efc38f-4e03-4e79...

 

If you want to collaborate with them, share files etc. create a team in Teams and invite them as guests.

https://support.office.com/en-us/article/add-guests-to-a-team-in-teams-fccb4fa6-f864-4508-bdde-256e7...

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