The invitation is an email with a temporary password, sent when the user is created. If it didn't arrive:
Ask them to check their spam folder.
Check their email address in Settings > Users. If it's wrong, create the user again with the right address.
Ask them to go to the sign-in page, select Forgot password?, and enter their email. Because they never signed in, UDP sends them a new temporary password.
If it still doesn't arrive, contact us with their email address.