How do people find out about local meetups like this?

I'm waiting for a movie at Cheonho CGV, and the people at the next table seem to be part of a local meetup.
They look to be around the 1975 age group.
It seems like a woman just joined; when one of the men asked where she was from, she said Gunja.
The other two men and two women seem to already know each other.
One man has dyed his hair yellow, and another man has sunglasses hanging behind his neck.
They are even sweeter than the caramel popcorn they're eating.