I consulted this with one of my superiors a couple of times in the past. I don't know the legal mumbo-jumbo behind it, but the bottom line is that having the game on your account gives you the right to legally play a game from GOG.com. In other words, you can install it anywhere you want, as long as it is you who plays it.

As far as I understand, the same thing applies to retail versions - you essentially buy a license to use the product and the CD/DVD/BD is just a medium to store that product. Borrowing an original disc from a friend apparently doesn't make it legal. ;)

Unfortunately we have no means to offer multi-user licenses, therefore we can't support schools, cafes, LAN parties etc, unless of course everyone who wants to play buys his/her own copy, which isn't very practical.