Like many here, I'm definitely an introvert, but I can act the part of an extrovert when I need to. I recharge on my alone time. I function pretty well in groups of all sizes, but I need breaks to regain my energy and focus.
Shyness is a different thing. I can be shy -- I was quite shy when I was young, and I have my moments now, like when I go to an event by myself where I don't know anyone. I really need to prod myself to go out sometimes because my comfort zone is at home with a book. But once I'm out, I'm rarely sorry I made the effort.
I like being part of a group, but I also tend to do a lot of things alone that other people prefer in groups or with a partner -- running/cycling, travel, etc. The first time I joined a group for a photography workshop, I was quite nervous about the prospect, but my excitement for the new experience outweighed any anxiety.
My mother was very, very introverted. I think my father was too. My sister is also. My husband seems much more extroverted than I am, but he has his moments of introversion too.
I test as an INFP. I'm not very extreme on any of those spectrums, but I'm pretty decisively an "I."