Hi @Jake Andrews , This issue may be because the user's email is not from your tenant. You said you added the tenant email to their username, but how does it appear in the portal? Please review this thread for details. Does the user have a work or school email?
The user must first sign up to register for your tenant.
Please let me know if the above helps.
If this answer helped you please mark it as "Verified" so other users can reference it.