To add a faction enter SM mode, go to the F9 screen, select a candidate planet to be a home world, and then click the 'Create Empire' button. Reseting the planet's minerals first with a rigged generation using the 'HW Minerals' button is also recommended.
A new empire may be assigned to be either player controlled or AI controlled. However, placing an AI faction on the same home world as another faction is highly advised against: AI factions are known to quickly resort to weapons of mass destruction heedless of nuclear winter risks.