On the left hand side of the screen click the 'People' tab.
Now click the 'Staff' tab at the top of the screen.
Click the 'Add staff member' button on the right hand side of the screen.
Now you will enter all of their information. Be sure to use their work email address instead of a personal one. Once you enter their information you will click Continue. Do NOT click 'Portfolio owner'.
On this page you will select the properties that the staff member is allowed access at.
Click 'Done' and you have added a new staff member.