Now I have more people who want to do this than I have slots to fill.

Three things that websites are good at: Holding more information prettier that a craigslist post, allowing lookie-loo-s lots of time to come and go anonymously, and maybe building your credibility if it is a nice website that provides value.

do you charge by the hour only or do you charge in 15min slots?

If there are pipes that go through the floor, you'll have to cut slots to allow you to slide in the board around them.

I now schedule 2 or 3 people for the same time slot and ask them to wait if multiples actually show up.

Since you say there are two doors underneath, a possibility (assuming this structure is level with the rim joist) is to slot the rim joist and cantilever 12-16' 2x8 or 2x10s to take the point load at the house, and then locate the outer beams properly.

I don't see an upside, it sounds like gambling to me, and I hate flushing $$$ into slot machines.

I earned some hours when I was in ROTC and hoping to earn a pilot slot.

I guess there are only so many "slots" or agents they allow for any given area.  

The only slot available was 7 am until 9 am before my first meeting.