They’re organized groups like teams, associations, scouts, or businesses. They’re also casual groups like friends with a regular lunch date, mom groups, a meetup, or a knitting club.
By their nature, they are people who like to connect with other people.
If you’ve got an extra conference room or banquet room, give it away. Get as many people in there as you can.
Everyone will be talking about what a great company you are. They’ll tell their friends how you helped them. And they’ll buy stuff too.
(That’s a lot of reward for space you’re not using most of the time.)