Hi Judith, welcome to the Community!
Assuming an email invitation was sent for a user, and they haven't been removed as a user from Confluence, they can trigger a "fresh" link if they click on an expired one. You can also click Resend invite in the Users list to trigger another email from the administration side.
If you remove an unverified user from your site through the Users list in administration, even a non-expired invitation link will not work to grant access to your site.
Hope that helps!
Daniel | Atlassian Support
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