Yes multiplayer is possible, but the practicality is still up for debate. To add an empire to go to the System Detail window (F9), select a planet with a potentially inhabitable atmosphere (i.e. Earth), and click 'add empire'. Make the new empire player controlled (and 'human' if Earth starting) and you are done.
From that point changing empires only toggles point of view. The AI will never make issues on your behalf so the game will only process orders you had previously queued up. In practice you issue orders for Empire A, switch empires, issue orders for Empire B, and then advance time. Having a dedicated SM/referee is useful for deciding how much to advance time, which interrupts are important, and optionally issuing new tactical orders to keep the game moving.
For further detail here is a link to a multifaction lets-play I started a couple years back. It didn't last long but it covered basic setup in excessive detail.
http://www.giantitp.com/forums/showthread.php?328883-Solar-Strife-Lets-Play-Aurora-4x