Huehue, I'll admit the whole sledgehammer thing has made me chuckle.

But I was thinking in that getting everyone together: like Whisk suggested, everyone would know each other to some degree, especially those of the 5A category. Now seeing as Mina and her father are arranging the journey, naturally they'd want to gather the right sort of people for it.
Could be that a rumour is circulating that has been leaked purposely to see who would be interested in the voyage.
And that Mina also goes looking for our character specifically for their skill sets:

We have a couple of scavengers/tracker types like Vi and Vale.
The Mechanic, we have The Hammer - mechanical specialty and muscle.
And our two military/security ladies who would be privy to the rumour specially because it's circulating rather quickly among them, and that they're ordered to look out for those who show interest? Which in reality they're more so interested in it themselves.

These are just obvious thoughts from the top of my head in how everyone gets to interact with Mina to plot the entire escape.