There are some horrifying examples being given of exclusion, and all of us need to stand up to this.
For me, it helps to have the attitude of treating everybody well and showing friendliness. I am a little suspicious of big groups of 'friends' being formed because they are a clique or sub-set of the whole group and why not have the whole group - e.g. mums at the nursery as friends?
I have tended to like to join organisations in my spare time, contributing to community newsletters, neighbourhood watch or being a school governor. My interactions have been about a purpose. I do have friends of course, for companionship, but prefer 1-1 when socialising with them. I'm quite good at taking in the detail of what people are telling me and like it when we give each other advice. Groups just dilute the listening and understanding process. Good for going to a concert or something, but that is bonhomie rather than friendship.
Any group which has a tendency to be excluding would be something I would avoid.
As I mentioned before, seeing all of us as a probable mixture of heritages means that I don't categorise people. I think that people who have experienced intermittent racism but who have persisted with following their goals are strong, stoical and positive and so I admire them and would be more inclined to be their friend. I tend to be feel most warmly about those who are kind to my son, who has autism and who has always been quite challenging in his behaviours. Kind people who run cafes and who kindly accommodate his requests and foibles mean more to me than, well, even family members who don't.
This forum is Mumsnet, so the majority who read posts will be mums, and this, for me, is a sufficiently common bond of shared emotions to use as a base for friendship. Equally, friendships with people who don't have kids are based on sharing a common aim to be nice to people and to make the community a better place.