Click the 'Principals' link in the top navigation bar.
This will display the Principal Browser. Select the directory in which you are interested, then click the 'Search' button to list all the principals that exist in that directory. You can refine your search by specifying a 'Username' and/or 'Email' (note that these are case-sensitive), or 'Active'/'Inactive' principals. (An 'Inactive' principal is typically someone who has left your organisation.)