The Created By field is a reference field for entities (Requirements, Scripts, Script Assignments, Automated Test Assignments and Incidents) and identifies the user that created the entity.
All entities created by user "username1":
CreatedBy = username1
All entities created by "username1" or "username2" or "username3":
CreatedBy IN [username1, username2, username3]
All entities created by any of the users belonging to the group "Testers", or the project manager of project 'Project X':
CreatedBy IN [MembersOf('Testers'), ProjectManager('Project X')]