Hi K.,
I do know what you mean: some people never outgrow their childhood issues and continue to behave as they did in high school.
Yes, some people are very close and do lots of things together socially and outside of school. We can't be best friends with everybody. But yes, it can be hard if you feel left out.
My advice would be to continue to step up: those women will embrace you for your generosity and team spirit. I'm yet to meet anyone who truly doesn't want assistance (but I have met lots who want all the glory anyway!) or a PTA who truly doesn't want more volunteers.
What I didn't do, was join the country club when I realised that there were a few women whose connection revolved around their membership there and their kids swim teams. So their conversation is always peppered with talk about what they did yesterday or will be doing tomorrow etc. But I realised, they are not doing it to exclude me, or anyone else for that matter.
I worked harder for my kids' school as I saw the benefits of my actions for the school community. I worked harder at getting to know them better, as opposed to waiting for them to get to know me. In time, I invited a few I wanted to know better to a movie: who isn't up for a night out to see a flick? Or have a kid free drink?. I've held a Bunco night, gone to lunches with them, the theatre, and now we have a book club that has grown and been going for almost a year now. And I've discovered what wonderful, generous, kind and hardworking women they are and have made good friends with a few.
In all of this, I've always included my established friends, too. Additionally, there will be other women feeling the way you are. Find those women who want to have a friendly familiar face at PTA events and be a friend to them.
In terms of developing the relationship between your child and his friend, there's a couple of thoughts I have. Have you spoken to the mother, as opposed to just emailing? Talking to her might yield a more personal response: maybe she is just very busy, or her son is less enthusiastic about a playdate? Maybe her child has so many commitments, she is reluctant to pick up something new?
Keep the smiles and friendly chat coming. +Good luck!!