UPDATE: The get together turned out to be a surprise success! About 20 people showed up plus two dogs and four kids, which Sister and BIL provided a slip-n-slide for the kids. I'm not sure what everyone brought, but we ended up with more than enough food and drinks that included burgers, hot dogs, variety of chips, two kinds of potato salad, fresh fruits, cookies, and a cake.
The get together turned out to be a 2) Everyone brings something, including meats, for everyone to share. This would require someone signing up to bring enough hotdogs for the whole group, etc. type of potluck, but it was simply poorly worded initially. A day or two before the get together, Sister wrote: As of right now, someone needs to bring hotdogs and "sides"! Cookies, salad of some sort, and really that's it haha...Any dish you feel is your "go-to" for gatherings! We loooovveee food! Oh and Friend, are you bringing the condiments for burgers? If not we will grab them! BIL and I are signed up for a fruit platter and chips as of right now though! It's supposed to be about 85 degrees so it'll be cooler than lately =), so this implies that it was a sign-up-to-bring-something potluck.
What also made it difficult to figure out initially was that the guests invited didn't know each other well or some at all, so it wasn't a "norm" in our social group to have this kind of get together since we weren't a social group. Everyone had fun though and suggested we do it again!