Having as many activities outdoors as possible, especially while eating, drinking, singing, shouting and exercising.
In indoor spaces by:
keeping windows and doors open, ideally on multiple sides for cross-ventilation.
running fans to increase air flow at windows and doors, pointing away from the group.
running HVAC systems with MERV 13+ filters to clean the air.
running HEPA air purifiers at the maximum rate tolerable starting at least an hour before gathering, during the gathering and at least an hour after to clean the air. Here’s a DIY HEPA air filter design for a lower cost ($60-80) option.