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.
For information about adding people, see Adding people and enabling them to log in.