I know we have quite a few members here from Illinois; and as I said, now is one of the best times of year for most of the country to do a get together....
A couple of ideas for you-
Set a good location first; this gives people a destination rather than just an unformed idea, and it helps with travel plans. get all the info about range fees, hours, what you can and cannot shoot there, etc;
Set some tentative dates; this helps people schedule; you can narrow to a solid date once you know when the majority of the people who are interested are available.
Arrange or figure out a social aspect to the get together; in all honesty, nothing works better for this than somebody with a pickup bringing a grill, and somebody grilling a bunch of good burgers and dogs and sausages, etc; you can make a tailgate/picnic of it, and it makes it a lot more fun and social, and it makes for a nice addition to the shooting. This is where location comes in; if possible, you want the location to be one that accommodates this sort of thing.
The shooting is great, and one of people's favorite things, as it turns out, is to try out someone else's gun; something they don't own themselves and have often wondered about shooting; so bring a little ammo for the next guy, and so on; it's one of things that make the events memorable.
I am sure I can think of a couple of other things, but in the meantime, if you need anything , let me know....