How you do it is up to you. I would provide the basics for the sandwhiches (ie mayo, mustard, lettuce, pickle, tomato) and then ask everyone to bring their favorite sandwhich filling (turkey, roast beef, tuna, egg salad etc).
We did this for a pizza party once, I bought crust, sauce and cheese. And asked everyone to bring a topping. We got a lot of pineapple (which surprised me), some pepperoni and olives.