Officially account sharing is not allowed:
GOG FAQ:
2. Can I re-download my purchased items? Is there a limit to the number of re-downloads?
You can always re-download items bought at GOG.com via the “My account” page. Also, there is no limit to the number of redownloads, but please remember that you're not allowed to share your GOG.com account with other users as only you are entitled to download items from your account.
I think it is ok if family members play the games when they are living with you since you are allowed to install the games on as many computers as you want in your household
9. Can I enjoy my purchases both on my laptop and desktop computer at home?
Yes. We do not limit the number of installations or reinstallations, as long as you install your purchased games on computers in your household. So yeah, if you've got a render-farm in the basement, you might actually break the world record for the number of legal Witcher installations in one household. However, if you think about installing your game on a friend's machine or sharing it with others then please don't do it, okay?
So I would not share the account with someone else, but would think it is ok, if family living with me plays the games I installed on our household computer. Of course I might be wrong, this is a lot of guessing based on the FAQ :D