Once you have spun up a new site, you can add more people to the site's dashboard. Use the person's berkeley.edu email address if possible. If the person has not already signed up at Pantheon, Pantheon will send an email inviting them to join.
Anyone you add to the site dashboard will be added as a Team Member and be able to perform operations on the site via the Pantheon dashboard.
If you are working with a third-party vendor to develop your site, the third-party vendor can be added as Team Members, but your department is still responsible for all activity on your Pantheon...