To set some expectations, "Happy Hour" is going to be difficult to do at Yellowstone for several reasons.
In most cases, setting a time for Happy Hour which most set at around 4pm, is too early here. People who have traveled a thousand miles are not going to want to be restricted to a time to be back, traffic and long distance to travel across the park being the key here.
We are never back from the park before 7pm simply because the later afternoon hours are better for reduced tourist traffic and the wildlife are far more active late in the day.
If someone wants to "own" a Happy Hour and preparing food, they can do so, but our past experience in places like this is that Happy Hour is poorly attended and most folks will have some difficulty juggling preparing food and returning on time. We've seen people grumble because they feel forced to cook every night for everyone and they don't feel comfortable not bringing something so we lose their participation in the conversation.
We were considering, instead, two different things, one a morning coffee session and sendoff, and possible a second "debriefing" at about 7pm in the pavilion for those who wish to participate and share their day's adventure. Certainly, bringing your dinner over "picnic" style is an option, but the happy hour/potluck thing has to be very voluntary.
You will quickly find that Yellowstone time does is not measured conventionally. So we are very purposefully not setting schedules to a clock and over planning folks' time. If you are pooped from "having" to do stuff, the rally will lose its value.
The goal here is to integrate new folks into the group, introduce those who are new to the park to all things Yellowstone, and exchange thoughts and ideas informally. So this rally is far more free form than most people will be used to. Yellowstone is hard to even begin to scratch in a week; we have been here twice before, for two months each time, and still have things to do. We want folks to have their appetite, whatever it might be, photography, hiking, fishing, sight seeing or just being with like minded folks satisfied.
But as far as "expecting" folks to bring something to a happy hour, there are no expectations. If one or more happens, it will be the right thing at the right time. 8)
Kim