(This maybe my own experience). First my apology to write this one in English.
After 4 months living in Seoul, I noticed that it is really hard for a foreigner to get close into a group of Korean friends. In another party, i was discussing with my roommate and some of my Korean friends. They all admitted that they do not want to introduce a new friend into their closed friend group. They explained that it would create an awkward moments for their closed friends, since it would take them a lot of time to get to know another "stranger" member.
So would those explanations be true? Or it depends on the personality of difference person?