I have significant social anxiety, but do not have ADHD. All my life I have tried to avoid any social situation where I don’t know anyone. It’s quite different from what you’re describing, but has the same component of being very debilitating. I actually feel physically sick when meeting new people, and especially in a large group or crowd of people I don’t know. Mine revolves more around what people will think of me and if they’ll accept me.
Now, my daughter has social anxiety that stems more from not being able to read people and a lack of those types of social skills. She’s graduating from high school next week and only has a couple of friends, and rarely hangs out with anyone outside of school. We’re both introverts, but, as you said, there’s a lot more to it than that.
Here’s some further information on social anxiety and ADHD:
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism