It wasn't scheduled, so when the alarm went off, we all looked at each other for a few moments like "is the siren just on the fritz or should we walk out?" After awhile we decided we'd better get going so we all lazily strolled outside.
On the way, I noticed one of my coworkers in a fire marshal uniform, in the corridor, calling out for people to evacuate.
I realized this is a perfect way to prevent the bystander effect: just give a random person a "uniform" (a whistle, a badge, a hat, or a vest, anything simple like that works) and tell them that when the alarm goes off, they are responsible for telling people to leave.
It's as simple as that. Instead of us staring at each other trying to decide whether the group considers it serious enough to go along with, just having that person singled out as the "official" one was enough to get the momentum going.