1. Sometimes an "inner circle" will co-host a party but the other attendees are not expected to do anything except show up and have a good time, and maybe bring booze. This is common with roommates and in college.
2. What you're describing verbatim is a potluck. Potlucks in the US are popular among immigrant groups, family friend groups, or parties for clubs or associations. But ultimately they're considered a bit uncool/laidback and don't fit the definition of an American party. They're better described as "get-togethers".