PROBLEM: Even after adding appropriate access to Teams, an external account (such a volunteer, contractor, vendor, consultant, etc.) is unable to open or edit files in Teams. When attempting to open the file, Microsoft provides an error saying that the user must use a "Work or School Account".
SOLUTION: Navigate to SharePoint to grant access to the folder(s)/file(s) via email
1. Navigate to Teams Channel where access is desired to be granted
2. Click on three dots to the right of "Sync" in the Teams channel
3. Click on "Open in SharePoint"
4. Within SharePoint, click on "Share"
5. Add the recipient's email address and click on "Send" via email
6. The recipient should go to their email provider and access the files via the link shared in email