>>1
It all depends on the reason you do it. If you just prefer to usually be alone, and you can talk to people sometimes when you have to (small groups or people you've known a long time) and it doesn't negatively affect your life in any other way -- then it's just introversion, which is perfectly fine. There are various books out there that explain that in more detail, look for one called The Introvert Advantage.

If you do it because you experience anxiety or fear, that's different. It could be anything from lack of social skills/confidence, to full-on social anxiety disorder. If it's based on fear, it's best to face it. It could also be part introversion, part fear.

Ultimately, it's about what you want. Are you concerned because you're feeling external pressure to act the way you're 'supposed to,' or do you really want to join those groups but feel that something is in your way?