Hi @Dave Brouse ,
Please make sure you have permission on this shared folder.
Then browser to the site collection you should have access, edit the URL by adding the following string to the end of the URL:
_layouts/15/people.aspx/membershipGroupId=0
For example, the full URL will resemble the following:
https://<contoso>.sharepoint.com/sites/xxx/_layouts/15/people.aspx/membershipGroupId=0
Remove the user permission and re-invite the user, then compare the result.
Reference:
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