If you pay someone $$$ to log into your account and boost you in PvE or PvP and get caught most likely your account and the person who played it (if their main account is linked) will get banned.
Never heard of someone sharing an account with a family member/friend and when nothing untoward happens get banned. If you share an account with someone and after a period of time you disagree with something they did to your account and go to blizzard for help, then you're most likely unable to get help and/or possibly get banned. The whole point of it is to "stop" people selling boosting services or attempting to scam the system by claiming they were "hacked" when they just unloaded some gold off to a friend and want to get the gold back from blizzard.