To invite an admin to have project-specific access, head to the admin dashboard and open the Settings tab.
Under Administrators, click Add New Admin to view the add new admin dialog. From there, enter the email address of the person you'd like to invite and click Add as project admin. They'll receive an email from [email protected] with a link to create their account, or if they already have an account, sign in to view the project dashboard.
*Remember, if you just want to have them view the live site before publishing, you don't have to extend formal admin access. To send view-only access to an unpublished project, click "Share login-free access" under the Page & Survey tab: