After you get enough money and run a mission, you get quite a long bit of story progression about a large drug deal. I'm not 100% sure what triggers it, though, as I had over the required amount when the deal came on the table and had to run multiple missions multiple times over the supposed amount and then had the thing go through mid mission, if memory serves. IIRC I was doing a run either in city center or seaside.
That pretty much cements the cast and lets you hire them all from the hangout.