ChillPal

ChillPal is a big Meetup group (6,000+ members) that runs casual meetups around Seoul several times a week: a Monday-evening dinner for about ten people at a different restaurant each week, Friday-afternoon café chats, and a drop-in Tuesday Social Night at Mike's Cabin in Hongdae (it moves to Han River parks in the warmer months), plus the odd picnic or Seoul Trail walk. Joining is free and you pay for your own food and drinks, but dinners and café chats take only about a dozen people and often fill up, so RSVP early on Meetup. Cancel early too: for everything except the Tuesday Social Night, a late cancellation earns a strike (three strikes mean a one-month ban) and cancelling within a few hours of the start means an automatic one-month ban.