Full spec is
hereOutput is standard USB - 5V 1A. The thing's rated at 2000mAH. It kind of depends exactly how much power the rPI pulls but if it pulls 'full' USB power a fully charged one of those would power it for about half an hour, roughly, given my 'back of the beer mat' calculation.
I suspect you possibly could keep a rPI going with a bunch of them if you could figure a way to 'patch' power across between them without breaking power to the rPI. And you had some people who are happy to sit there winding. Given the manual recommends turning the crank at 120RPM and the charge time is 150 minutes, you're going to need six of them with 5 people winding and it's going to hurt.
It's all a bit mythbusters really

.