how to keep mac pc on when it's on file sharing?

How can I keep a Mac running when I'm using it for file sharing? It's my parents' Mac, and I use it as an external drive. When they come home, I usually close the lid, but that causes file sharing to stop. I've tried adjusting the settings, like turning off screen timeout and using a screensaver (which works but is noticeable). Any ideas?

Question by masterchief844 on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.