I'm personally ADHD mixed with autism. When they're so closely linked, it seems that medications do very little for ADHD symptoms so there might honestly not be much someone can do about it. Note - I'm mildly autistic (mild considering the possible extremes) and moderately ADHD. I used to be really bad, but I'm less hyperactive now (perhaps age and drugs help a little)
However - if someone is in my field of view and talks to me, I will usually notice them. Especially if we were in a prior conversation. To be fair, I'm not very aware of my surroundings, and I have below average hearing, so it can appear that I ignore people. (all my senses are weak, which is nice I guess for ASD, though photosensitivity still is awful) I also have huge problems modulating my voice, I'm always way off in one extreme or another.
On the other hand, if someone is focused on a game and you say, message them in game, message them on IM, text etc. then clearly theres a very good chance that they will miss it, with good reason. Also, as you may know, many people with ADHD and or ASD tend to be as*holes - whether its their fault or not. ADHD makes me fidget, squirm, get up, pace a lot which further affects my ability to socialize properly. This sends me in a downward depressive cycle which causes me to be more aloof, hostile, and lash out more at people. So in other people's eyes, I'm very much an as*hole - at first for one reason caused by my mental issues, but later on reinforced by the result of the said mental issues.