SCCA member here... yes, to put on a "real" autocross, we have agreements with the owners of the lot and most often times they require insurance (we've used a Ford plant, a vocational school and even an airport runway).
There are a number of insurance companies that will write a policy for stuff like this. It's not that bad, but would probably work out to be a few bucks a person just for that.
We also have people sign a waiver/get armbands when they check in...just to protect us, but then again, our club is associated / protected with the SCCA as a national organization. Not sure how a private event might work, but you want to make sure if someone gets hurt / breaks something, they don't come right after your head in particular.
One tip from putting these events together... make the riders also be workers. In our events, you run one heat, then work one heat alternating. Workers chase cones, help with safety / scoring / timing / announcing, etc. It's required. No worky, no runny. It does make for a long day, however, especially in blazing Ohio sun.
Probably more formal than you want but without workers, the event doesn't happen... lots of people keen on running their cars but left to their own devices, they'll leave and get a burger rather than help the next guy.
Depending on how you do things, you'll need to get / rent / buy cones or mark the course in some way. Perhaps the local SCCA guys might agree to lend you theirs. Plan on a truck / trailer for that stuff... they also have timing lights which are super cool.
If you want to talk more about how those events are organized, etc.... let me know. Sounds like a fun idea.