A contact cannot be an administrator.
However, we have a trick to use the email address of the contact to create an administrator account for him/her, using the same email address. To do that, simply add + sign in contact's email address to register the contact as an administrator.
So, when that contact logs in next time, he/she simply adds the + sign in his email address to log in as an administrator. Add + sign as in the example below. You can also add a trailing number besides the + sign.
Another example: email@example.com