I admit that to some my pronouncements on Density, Structure and Lighting may seem totally contrived.
Forgive the build up. It's really all about creating good Flow.
Flow is that feeling you have when a party is going well: guests mix, enjoy what you're offering, and maintain the buzz of many conversations. You know it when you see and hear it.
Having the right number of guests (density) can get people into each others' personal space so introductions and interactions are natural. Having your party space laid out well (structure) can help keep guests mixing and moving.
Lighting can help set the mood and cue people to relax. Good flow follows…