I'm guessing they spawn directly into any seat, but it's nice to know that the rooftops can have people (the pool and pool chairs would also be good to have people on). I'm interested to see if you can rotate a chair on all axes, because that opens a lot of interesting, albeit strange possibilities.
What do you think you'll do with the chairs? It seems like you could make a classroom, maybe a whole bunch of theme park rides that don't move.