Thank you. It’s curious: I saw that apostrophe in the error message too and dismissed it initially. I know Excel does that sometimes to text in a cell when you convert it (I coverted this from XLSL to CSV for import), but I’ve double checked the import file (in VS Code) and don’t see any leading apostrophes.
Thank you for providing the regex and clarifying that it should be allowed. I’ll have to tinker more to see if I can figure out why it’s failing. (It failed even when I deleted that user, removed the +test from the email, and tried to re-send the invite.)