How to share video file (DVD) in company's network to employees without them being able to take it home?

You should be able to convert it to a format that plays off a website, stick it on your company website or reverse-proxy it so it only allows IP's from within your Intranet to get the page with the link. That should limit who can view the video from your server.

Then remind people that if it leaves the company it's a firable offense by policy.

As others have pointed out there's always a way to get digital information and steal it, even if it's by pointing a cell camera at the screen and recording it. If it's extremely proprietary information you can't just serve it out to people on the network and not risk it being stolen to some degree.

Alternatively you could just put the files into a share with particular permissions to restrict who gets to it.

Depending on how far you want to take it and how much space you have dedicated to file storage, you could embed information into the files, like a pixel marking of some kind or a special numbering tagged into some frames, that are then compressed into a small video and set permissions such that only certain people have access to even list certain versions of the video. Then if it is leaked you know who's login was used to access it. It won't stop the leak but it'll tell you who is most likely responsible. Or directly give it to the users and tell them that it's been tagged so you know what version goes to who, and they have to return the USB drive or CD when finished (I don't know the nature of the video you're distributing.)

Last would be to show the video to people at a mandatory meeting and then not allow access to it again. It's really the best way to control whatever the content is. Anything else still takes the risk of theft in some way.