Every pit I've seen has been pretty informal. It's 1st come 1st served, no assigned spaces, and there's plenty of spots. Just show up before your race to drop off your stuff, and clear it out soon after your race.
As far as what to leave at the pit, it's normally just a spare wheelset and/or a spare bike.
Know that you will not be allowed to short cut or back track to the pit. So if you get a mechanical after you pass the pit or somewhere far from it, you will have to run your bike on the course all the way there.
Good luck Sunday!