I've always had trouble making friends, but I realised I've also shied away from opportunities to have friends, saying no to things because I was anxious about them. So I think you've done a great things accepting that lady's offer, really what is the worst that could happen? Things really improved for me friend wise (with my grand total of 4 friends now!) when I started saying yes to people, even to things I wasn't too keen on, just to have shared experiences with people, and you know what none of them have gone wrong like I was anxious about and I've had some great fun!
My AlAnon group is very small in comparison to yours, but there is the same thing of the longer time members being friends outside the meeting and I felt the same as you at first, but over time I've become one of the regular members and those friendships outside the meetings have started to grow. It takes time. And like you've I'm learning that it doesn't take lots of work to keep a friendship up, but I do need to keep in touch with a text or email or coffee to keep it going
Its great you have been able to have such insights into why you were feeling that way and then do something positive about it
Hope your retreat is very enjoyable for you!