accessing a private ip from another global ip

"If you want to access it without an intermediate computer, your only chance is to do some port forwarding on your router (port 22 is the standard one for SSH). Alternatively (if you have not that possibility) you could scan your router from the internet and find out what ports are already open. If there's one that is open and unused, you could configure SSH on your ""private"" computer to listen on that port rather than port 22.

Alternative methods imply the use of a globally accessible computer on which your private computer will be somehow permanently connected. You will have then to access this ""middleman computer"" in order to access your provate one. There are tons of how-to's online. Googling ssh to a computer behind a router will return about 1 million results."

Yes, but you would most likely have to access it via their "global ip" and set up port forwarding on the router to point to their private IP.