If you invite someone to join a team as a guest they will be redirected to create a free Microsoft account if they do not have one already.
If they are a part of a domain that is joined to Microsoft, they will be able to use the link.
To answer your question "Is there a way to invite these external guests to the team without requiring them to have a Microsoft account?" directly the answer is no.
Note: you can join Teams meetings as a guest without a Microsoft account.
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