Write down an e-mail explaining that they need to get the money to you and ask your boss to send it to everybody from his address, since it was a company lunch and it should ultimately be his responsibility, holding your money hostage by explicitly ordering you to pay isn't ethical either but since he doesn't want to give you the money, you don't have many other alternatives, and this is a middle ground which he may accept.
Asking everybody for the money yourself would be weird and put pressure on your relations, which isn't good.
I'm not sure any of this matters to your boss since he's making employees pay for a company lunch, but it doesn't hurt him financially to send that e-mail so he might do it.
There's also the chance he doesn't want to deal with employees being surprised that the company isn't covering it, so maybe he won't even do this, because he wants the subconscious emotional blame to be on you.
In this case evaluate the amount of money and depending on the kind of workplace, you could just eat it (I doubt it's feasible given that you weren't just 2 people). If your colleagues are understanding and you're on good terms, I'd ask rather than eat it.