I had a couple of 'behind the scenes' alters that never really took over and acted in the world, but had very important internal roles, nonetheless. I can think of four that had different roles like that. I wouldn't say they were complete alters in my case, but they were psychological constructs that acted independently of my awareness / me, if that makes any sense.
I guess my experience has been a little similar to that of Amanda-Louise... throughout my life I had alters that took over as needed but I didn't really become consciously aware of them until therapy. I had awareness of them and the havoc they created, of course, but it was in therapy that I first became very consciously aware of the first two that introduced themselves to me (and was able to recognize them retrospectively).
From there co-consciousness built up slowly over time, eventually to the point where we had a core group of 'every day' alters that could converse and carry on as we went about our daily lives. The dissociative barriers broke down between this group more and more until (years later!) they just weren't there anymore at all and we were operating / feeling / existing as one.
Now I 'observe ' myself sometimes and fondly recognize behaviors / feelings / actions that would once have belonged exclusively to x or y or z.
So yeah. It was a process of not-knowing - recognizing - communicating - co-habitating - becoming.