In Skills Base, "People" refers to all individuals within the system. This includes both people with login accounts, as well as people without (who may have been added manually by an Administrator). All People can be assessed in assessments, however Administrators can control exactly which privileges people are granted via Security Groups (See: Configuring Permissions).
The People Directory (Directories -> People) contains a list of all people in the system.
From the People Directory, you can apply filters across entities that have been assigned to people (Team, Role, Location, Skill Set, Security Group).
Simply click the filter icon on the header of the column that you wish to filter on. You can filter on multiple columns at one.
For information about adding people, see Adding people and enabling them to log in.