If the person has not already signed up for your nation, go to People and select New Person. Enter the person's name and email address. Then change the Permission level. Be sure to save an initial password before clicking the "Add person" button. Tell your admin her initial password and remind her to change it when she logs in by editing her profile and going to the Password section. 

If the person already exists in your nation, edit her profile, add a Permission level and then click the "Save person" button. Learn more about what permission levels in your nation mean.

Below is an image of where the permission levels appear in a person's profile.