For example, if you have a golfing group, you guys may go out for drinks but that would be that. You'd almost (key word) never go over to someone else's house, or go see movies together, or any of that -- they're your "golf people."
But that's assuming that you do have a golf group or whatever. A large number of people just go through life with their friends from elementary, junior high, or high school... and then their friends from college. Then friends from the workplace, if you make any beyond the level of "drinks after work."
I know a lot of people who have "drinking friends" from various adult social circles, but their only "real" friends (in the Western sense of people you can chat to about almost anything, do almost anything with, etc.) are a tiny handful of people they've known since junior high or high school.
Loneliness is a huge problem in Japan.