Honestly, I don't think you need to go into much detail. I missed a year of high school due to other psychological issues and came back with a lot of accommodations (so it was obvious to everyone why I had extra time and such). However now that I'm in college, I had to make the decision to tell people. I understand that it's a fine line - it's something personal that you don't feel like sharing with everyone. For my first exam, I just told my friend (who was in the same class) - "Hey, good luck. I'll be taking the exam in another location, so don't worry about saving me a seat." Of course he asked why, and I just told him that I had a learning disability and it was a testing accommodation. And that was it. Most people will have the respect not to pry too deep or ask for details. I have ADHD but I usually use the term learning disability (because although it's not technically a learning disability, it certainly affects my ability to learn in the traditional academic environment). I've found that saying "learning disability" instead of ADHD leads to a lot less questions being asked. People understand the concept of a learning disability. However the term ADHD/ADD can raise some serious debate among people who deny that it is even a legit disorder. Then you find yourself getting into a debate about whether you truly need the accommodations and whatnot.

I guess if I had to give some specific advice:
- Share only with people that you feel comfortable sharing with. You don't have to explain yourself to anybody that you don't want to (besides like, teachers and the counselor and stuff)
- ADHD/ADD is not something to be ashamed about. It's just a cognitive difference. And a good friend will accept you for who you are, even if they don't understand ADHD/ADD.

Oh just one other tip - I would avoid saying "personal issue". I did that for a while and you have no idea how many rumors came back to me. I heard talk that I was dealing with everything from mono to a cancer diagnosis. If you leave it so vague, people will usually assume the worst. If you are going to choose not to tell them, I think it's a good idea to simply say, "I have indivudal circumstances which allow me to have testing accommodations. Don't worry - it's nothing serious. Please understand that I do not want to go too much in detail". And any polite person will not ask questions.