Can you share Steam game files? [duplicate]

Yes

I did this multiple time in college (1 year ago). Some of my friends had a really slow internet connection at home so I copied the game files on my portable HDD and transferred the files to them. They then copied it in the right folder and when clicking "install" in Steam, it discovered the files.

Doing this with games coming from a shared library also works, so you don't need to buy a game they have in order to download it for them.

You said in the question comments that you don't want to risk getting banned from this. There is no danger at all. You are not breaking any DRM while doing this. Your friends still need to buy the game if they want to use those files. If they don't own the game, it wont show up in their library.