I'm a little bit of both! ~ For me; it depends on how I'm feeling that day, who I'm joining in with, and if I've had plenty of caffeine or not. ;)
A lot of the time I prefer to be the loner. I don't really join in with groups unless they're a group of close-knit friends. I have three people I consider close-knit friends; I prefer their company over the possibility of a hundred mere acquaintances. I'm very introverted and too much socialisation can make me feel so incredibly tired and I have to retreat from the world to recharge my batteries, so to speak.
Sometimes I feel the urge to want to get involved with some groups, even if it's just a local group of people who enjoy photography, but I never do. Maybe one day! :) (not today. ;) )
I hear ya. I sometimes think something sounds like a good idea then don't do it.