It's a loose RSVP. A few extra folks showing up last minute is not a big deal, the meals are usually "family style" and leftovers get taken home most of the time.

If someone is rolling in with a larger than usual group they typically bring enough extra food to go around.

And yep, a lot of these are pot luck style to some degree. Host makes main course, everyone else pitches in on other stuff.

Definitely some loose coordination involved and the host will typically call a roll call the day before to get an estimate of number of people dropping by.

This is what potlucks are for.