A pop up picnic is one of the simplest way to gather your people. A day or two before the magical evening, send invite to everyone you know who is local. No trying to figure out who would make a good mix - invite them all. And ask them to invite all their people.
Pick a public setting and ask everyone to bring something to share.
That is it.
Whomever shows up will be the exact people needed to make the party the best.
Try it out once a month or so starting this week.