I should hope it’s not a sin, as I used to be really withdrawn and shy as a child around groups of people or people I was unfamiliar with. I think that if that was/is the case for you, then it’s not a sin.
I think the what you’re referring to is that we are called to be open and to spread our goodness and faith. But I’m sure that is being told to us with consideration for people who have social issues. Even if it wasn’t imagined that way, it would be in a practical world.
In casual situations, at parties and such, the best thing is to just be yourself and try to find someone with similar interests or have a friend introduce you to someone with similar interests. Avoid going alone to social outings, so that you’ll have backup and support on-scene in case you get a panic attack or something.
Alot of socializes IS all external, and to be frank, I’d avoid being disingenuous since many people just put on a mask or act and getting mixed up with them could be a bad idea. Body language is sort of important, but not as important as most people put stock into. Alot of people say it’s about confidence, but I think it’s more about understanding and communication. You need to be talking with someone on your level, and humans are pretty good at gravitating towards these people. It’s also about observation. Look around to see what’s going on around you, and listen to people for a few moments, and if you see an opening take it.