All you do is visit this site, and choose between "Share" or "Join". If you want to 'share' your screen with another person (so they can help troubleshoot your ArmA problems) you select the option to 'Share' and then download a small little application. Run this application, and it will give you a code. Now all you do is give the other person this code so they can get access to your computer screen (and move your mouse around, if you give them permission).
The other person, wanting to see your screen, visits the same site and just selects 'Join' and puts in your access code you supplied them earlier.
You (if you are the one sharing your screen) will be notified when the person connects, and if they then ask to control your mouse, you will be prompted to accept/deny. After that, the other person can then move your mouse around and help you troubleshoot your problems, fine-tune your ArmA settings, work out what drivers you might need to install, etc.
It might also be a good idea for both people to jump on TS3 so you can chat to each other if anything goes wrong, or someone has a question, etc.
Note: Join.me does NOT install any software on anyone's PC, no matter if you're the one sharing, or joining.
So if someone is having problems with their PC, or ArmA, consider volunteering to help them out via this site (instead of spending pages of posting to diagnose and troubleshoot their problems).