Unable to add developer to private project

I invite a developer with the Members option. Invitation mail never reached to developer’s email address. Why invitation doesn’t reach to developer’s email address?

Maybe it is in Spam folder or it was blocked on the way by Corporate filters if it is a company’s email.
Emails are not guaranteed delivery method. You can try again.

What is an alternative to invite a developer to the private repository?

Developer can create an account on GitLab.com and you can search for him by name instead of sending him invite.