This really becomes a sore spot with me when I talk to certain Witnesses who seem to think I have something wrong with me because I am introverted. It's not an illness, it's a personality type! I don't want to eat out with a group because it's a very uncomfortable experience, not because I hate people or need to "widen out"!
It's funny you mention temp jobs, Mum; I had good experiences working as a temp between regular jobs. You'd think that introverts would hate temping because it means having to keep meeting new groups, but I think in a way it actually makes it easier to interact with each new group because you have this clearly-defined relationship -- you're not joining their family, you're just coming in to do a job and then go on your way. So there's no wariness on the part of the regulars -- you're not replacing anyone, and it doesn't matter if you're a bit weird because you'll be gone soon anyway. I tended to feel welcome in each new place I was assigned to. And as you said, once the agency sees that you can do a basic trial job without screwing up or suddenly failing to show up, you're like their new best friend and you get tons of assignments offered to you.