Members of the in-group are often blind to subtle things that are very exclusionary of members of the out-group. An example that doesn't bring racism or sexism into this: I joined a forum some years ago where the owner routinely lamented how he desperately wanted more international members. He was probably one of the biggest offenders in terms of creating an exclusionary environment that made foreigners feel unwelcome. One piece of the culture that was very obvious to me as an exclusionary practice was the on-going joke that if you were on the forum at certain times, you were a total loser with no personal life. These were the same times that people in other time zones would be available to be online (ie people in other countries).

At the time, I happened to be frequently up all night. I made it my goal to actively root out this piece of forum culture and "set an example" about how I was online all hours. About six months after I joined, membership began to go up quite a lot -- but especially international membership. I'm sure that to this day the forum owner has no clue whatsoever how that happened, even though I have stated that I made it happen. I doubt I am believed.