Quite often friendship groups are based on a common interest, rather than deep friendship.
When I had my children I lived on a new estate and we used to joke about the Baby of the Month club, a group of us, all living in the same cul-de-sac became good friends. Helped each other with babysitting, ran a common maternity clothing wardrobe, had coffee and tea parties for adult company and talk. But once we started moving out and on, we really had very little in common apart from our joint maternity and living in that cul-de-sac. I still swap Christmas cards and a letter with 2 of them, but we haven't met in years.
At another group involved with my children, I did meet someone who became a close and very dear friend, and I was devastated when she died suddenly.
I think, as with most things in life friendship is a line, not a point and friends can be both loosely held and almost a sdeep and committed as a good marriage.