BioWare has uploaded a video of the multiplayer demo that they showed to press at E3. Here's the description:
"Commander Narlock readies his troops for an Imperial assault. As he does, all four Republic classes; a Jedi Consular, Jedi Knight, Smuggler, and Trooper join together to assist in fighting off the attack. Watch as each player does his or her part to take on the enemies, and beat the Imperial Troopers and Imperial Breach Droids."
Watch at the official site