This is pretty much me too. I have a small group of local friends (who are great - there are 4 of us) and a handful of other good friends (well, 3). I have had to really really work at this though and make lots of effort as I'm someone who just gets overlooked otherwise.
My teenage son was diagnosed with ASD last year and now on ADHD pathway and it dawned on me that I almost certainly have both too. On paper I seem to be a lovely person and ideal friend but socially my wavelength is a bit off (and then I get shy or anxious) and people think i am weird/hard work/ annoying/boring. It is evident that even my own mother feels this way about me:(
Primary school - had a best friend - she was always MY best friend. I was not always hers. It was a very small school so whole year groups tended to play together so its hard to say if I was in a friendship group but, thinking about it I was often awkwardly on the periphery and often didn't get party invitations.
Secondary school. I did become part of a group of 6 early on but in later years it evolved and I didn't! Other members of the group made friendships with other individuals or groups outside ours but I had no idea how. I did have a best friend within the group but by the end of school she was best friends with someone else (I wasn't).
I joined a church youth group - most of them were cooler and more streetwise. Again I was on the periphery. I was (boast alert) quite pretty and went out with some of the guys. Most of the kudos I had came from being so and so's girlfriend. When the group shut down I barely saw them though they still hung out together.
Made contact with one of the school group who hadn't stayed for 6th form and we used to go out to pubs and gigs. However if her sister was around, or she got a boyfriend I wouldn't see her for dust. I had several boyfriends during this time though a lot of the time I felt that I'd rather be hanging out with a group of friends but I didn't have one!
Uni - Made lots of acquaintances in the first term, went to everything I was invited to, made lots of effort. Joined up with an established group and was so happy. I then got together with the alpha male of the group, great at first but soon after I became invisible, just X's girlfriend. He was an extrovert and I wasn't. I tried really hard to forge closer bonds with others within the group but my perceived awkwardness and desperation was off putting I think. We left uni, we split and the group kept in touch with him and apart from one tried to drop me.
Workplaces, similar stories. I made one lovely friend at my last firm (still friends today), and a few acquaintances along the way. I wouldn't say I have any friends at my current firm though I have tried. I am senior now (and married with a family) so it bothers me less than when I was a junior and getting left out of drinks and get togethers that people arranged weekly. When I did go I seemed like a fish out of water.
My group of friends I have now were acquired through a lady I met at ante natal group with ny eldest. I really liked her and put in lots of effort to keep in touch. This approach often scared people off but she took to me, luckily. The other two in the group were already her established friends. It is a lovely group but even so I reckon if they were each to rank their favourite people in the group I would rank last. At one point someone else joined the group and tried to freeze me out. This went on for some months until she dropped out again. At another point I proudly introduced another friend into the group who promptly dropped me like a stone once she'd met my cooler friends!
I'm ok with it now and its reassuring to read threads like this and know I'm not the only one. It does make laugh when people post about disloyal friends and mumsnet says "just make some new friends". If it was that easy we'd be doing it already!