I have an ADD diagnosis - however superficial - it consisted of a couple of screening quizzes and a prescription for Concerta. (Essentially slow-release ritalin)
So I can tell you a little bit about it - because to some degree, I found it fits me. However, when I got the dx 3 years ago, I already had my suspicions about having AS/ASD.
That being said, I can tell you (from my own, very subjective experience) where some of the overlap exists:
Keep in mind, I am most likely in the inattentive subtype group and not ADHD - so that means I have always been quiet, shy, kept a low profile. I don't have a lot of the impulsiveness that you would have with ADD. In fact, very little.
Hyperfocusing - is very possible. I do it a lot, but less so on Concerta
Stimming - can be attributed to hyperactivity. Fidgeting, scratching, etc. is very common, even self-harming. I've found it lessened by Concerta (till it wears off)
Social anxiety, communication problems, visual cues, etc - could also be due to attentional difficulties - which seems to be the case for me. OR it could be sensory overload. (Pretty sure I have auditory processing issues.) 6 of one/half dozen, etc...
Obsessive & ruminative thinking - common feature to both, as well as OCD - which I was convinced I had for some time. (Note I'm also dysthymic, which likely stems from this.)
From what I understand, both conditions have much to do with executive functioning and working memory - but ADD seems to have more to do with problems with planning and strategizing. Autism, not so much.. (Or so I think?)
Hope that helps a bit... I'm guessing I might have made it more confusing though..