If they are friends of Joe, will they be addicted to his presence?
This is complicated.
The first thing you have to take into account is that the happy people around Joe will have an unbalanced amount of some neurotransmitters related to the feeling of content, joy or whatever. I am not a doctor, but to me this seems similar to having taken some kind of antidepressant drug without having any medical need for it.
Science itself does not have a definitive answer about whether antidepressants are addictive:
Antidepressant dependence can form in people who never needed the drugs in the first place. Some people are incorrectly diagnosed with depression and prescribed antidepressants. According to one study, doctors misdiagnosed almost two-thirds of patients with depression and prescribed unnecessary antidepressants.
Doctors still debate the addictive nature of antidepressants. Most consider these drugs non-addictive. Others point to the withdrawal symptoms of antidepressants as evidence that a dependence can form. People who suddenly stop taking antidepressants often have withdrawal symptoms such as nausea, hand tremors and depression.
To complicate things, your character is not giving people around him a dose of antidepressants - he is giving them pure neurotransmitters, so it's even more difficult for us layspeople (and I think even for doctors too) to predict what would happen.
Now, while we don't know whether feelings of joy would make people addicted, I think feelings of sadness would. If you ever knew someone who was a goth or emo as a teenager you know what I am talking about. I lived close to a place which was a magnet for emos. I could hardly ever feel sad myself because just looking at them made me feel [expletive] happy by comparison, and that emo thing lasted like a decade.
If they are captives, will they die of a heart attack if they receive a sudden shock? Or are neurotransmitters not this magical?
Whenever you hear something about someone dying from a heart attack due to some sad news, usually it's an elderly person who had some heart condition, and there are.other stresses involved; the sudden sadness is just the last drop that does them in, but there is always more to that. If sadness could kill through heart attack, every mother who had their child killed violently or unexpectedly would die too.