First, you need to add that person as an Admin. Log into your Admin account in BallotBox, and then click on your name in the upper right corner of the window. Select BallotBox Settings. In the page that appears, click on the Admins tab, and then click New Administrator.
All you have to do is to enter the name and email address of the new Admin. Click the Create button. The new Admin will automatically get an email from BallotBox asking him or her to set a password. (In the listing of Admins, you can click the Send Login Link button next to an Admin to re-send that email, but if you do, the link in the previous email sent to the new Admin will be automatically invalidated.)
Once the new Admin has set a password and logged in, he or she can click on their own name in the upper right corner of the window, pick BallotBox Settings and then the Admins tab, and remove your Admin account by clicking the Edit link, and then selecting Delete Admin.