No expert I.
Immediately turn towards office door and get through it. If no gun, get gun and extra mag from office. Reach defensible area with phone (or exit building and use cel phone) and call police: "armed robbery in progress." Cover door with muzzle until police arrive.
Reasoning: 4-5 potentially armed bad guys in room full of people. They're not gonna drop their weapons at your command when you draw your Sig or what have you, unless you're a Jedi Knight. The best gunfight can be the one everyone walks away from. If they start shooting hostages, well, that's another argument for concealed carry rights(joke). Seriously, I guess you'd have to do some mighty serious assessment of the situation if innocents started getting killed and you were the only armed human being there.