A little bit of both, actually! I have two set-aside times where I have 5 open slots for players. I also provide a list of rumors/jobs (it's organized around a neighborhood in a bronze agey megalopolis) - if 2+ folks want to investigate a rumor or take on a job we play.
Folks can bring whichever characters and play whenever they like. There's no real trouble joining late or leaving early during a session, either.
Does that make sense?