I'm not young (38), but my parents used to throw these kinds of open parties all the time when I was growing up. We had an open door policy, too, where anyone who showed up around a meal time was automatically served, too, and that provided a really awesome opportunity for us to build and strengthen friendships (being older and more worldly now, I know this is pretty common in large parts of the world, but it wasn't in central Virginia in the early 80s). We had a couple acres, and our house parties ranged from just a couple of folks over to hang out to large outdoor events with up to about 250 folks. In those cases it was always friends-of-friends-of-friends. As mentioned elsewhere, my family provided the venue and some general amenities (some drinks, core food) but it was expected that attendees would bring something to contribute.

As adults now, my wife & I try to have friends over regularly, too, but it's a lot more complicated in suburban US because of dual-income households, kids that either don't go to neighborhood schools or are involved in lots of extracurriculars, and generally antisocial neighbors.