The problem is, Yahoo refuses to recognize that email address as it is not a Yahoo mail address.
Um, I'm not sure what you're doing, but a Yahoo Group's Invite page does not require that the email address be a Yahoo Mail address.
If you're receiving an error message about being unable to load the content, you can probably ignore that, the invitation may have been sent anyway. This is a common failure in Yahoo Groups recently. ?
Please advise how to proceed.
When using the Yahoo Groups' Invite page include a test email address along with [email protected]. Then watch that test address to see if you get the invitation. Make sure to check the spam folder and anywhere else your email service might deliver the message. The test address must be an address that is NOT listed in your Yahoo account (not one of the ones you can select in the Edit Membership or Manage My Groups pages).