Locally we have these a lot. In fact there was just an entire weekend one this past weekend. While this is organized as a 501st event and mostly it is 501st armor that is worked on, people generally just bring whatever they happen to be working on at the time. It really is all the same principles and skill sets for the most part.
In terms of setting it up, I always set out the times right from the start. If you're good with having people there until the wee hours, it more than likely will happen. But if you'd rather get a good sleep, set a start and end time and then people can plan accordingly. Makes for a less awkward start and end.
When we do multi-day ones, we also organize food so the host isn't responsible for all of it. Have people sign up so that whoever is bringing stuff knows how many they are feeding. Have people mention allergies so that those bringing food are aware. It doesn't mean that everyone must accommodate each allergy but with the awareness there people can mark food accordingly. Snacks and drinks are also important as are disposable plates, cups and cutlery. This also saves the host a ton of time.
As the host, I find having big garbage bins set out from the start helps - ones for garbage and ones for cans/bottles. Also, let people know where they can do certain things. If you need them to only sand outside, just let them know at the start. Mark your own tools as they sometimes end up accidently going home with others.
If you have pets, let people know before they arrive. This helps people either drug up or decide they can't go. It also makes people aware that they need to be more careful of stuff they drop etc.
If work surfaces and chairs are at a premium, ask people to bring their own folding tables and chairs. I also always suggest that people bring indoor shoes so that no one steps on something and gets hurt.
Most of all, have fun and learn!