From expat groups to coffee meetups to language exchange, the choice is yours on how you want to integrate. It's a large city so there are always ways to get out there and socialize. A meetup or two with one of the popular expat groups will introduce you to plenty of people and language exchanges are excellent for meeting local Koreans as well.
LanguageCast Gangnam
A place for language exchange and meeting other foreigners and local in Seoul wishing to learn about other cultures
Seoul Players
English-speaking theatre group regularly meeting up rehearsals and socials in Seoul.
The Seoul Global Expat Meetup Group
Large Meetup.com community for regular meetups and networking in the city.
British Association of Seoul
Expat club for British and third-country citizens in Seoul. Regularly hosts events and has a strong network.
Wine and Women Korea
Large community dedicated to bringing wine and knowledge of wine-making all women in South Korea. Great for meetups and cultural exchange
Very Expat in Korea
Huge online community for expats living in South Korea. Great for networking and advice.
Seoul Coffee Group
Large Meetup.com group for coffee-lovers. Great space to meet others and network, with regular events hosted across Seoul.
Seoul Expats
Large online community focused specifically on Expats in Seoul.