In order to add Admin's to your account, navigate to the vertical menu bar on the left hand side and click on the "Sharing" icon. Next you have two options. You can either click on the orange "Share Cap Table" button to add a new sharing contact or click "Edit Access" for existing sharing contacts.
Next, select the Access level drop down menu and select Admin. Enter the appropriate information and click the "Send Invitation" button on the bottom of the page