Schelling points. Only so many can fit in a given bunker, so some people must be excluded; excluding all those who did not sign up in advance, and only those, is a very straightforward place to draw the line.

Another obvious Schelling point is to exclude all but those with the most political, social or physical power (whichever of those seems to be the dominant factor at the time). I don't particularly like that Schelling point but it is the one humans follow by default.