Registering Owners and Tenants for their portal accounts starts with an invitation. Occasionally an owner or tenant says they didn't receive the email. The first thing to check is the Email Verification report under Contacts.
This report will show you any email addresses that have bounced or were otherwise undeliverable.
If the email address is good, then you will want to try resending the portal invite and have the person check their spam/promotion/update folders.