If your admin username is the Site Owner or if it has the "User Account Administrator (Add/Edit/Delete Users)" privilege, then you can login to your site's admin area and create new admin accounts for your site.
After you login, hover over the Page Menu, then over "My Site..." and click the "Add New User" option.
Enter this person's email address, name, etc., and give them the desired access privileges (e.g. can update pages, can update products, etc.). Click the [Save Changes] button when you are done and the new username will be created.
Notes: