Is it possible to play a custom map with friends? I've seen people like Vanoss do it, but whenever I try to do this both me and my friend get a "unable to find usermap on workshop" or something like that, even though we both have my zombie map downloaded and loaded in Black Ops III.

Anyone know?

Just upload your map to the steam workshop, subscribe to it, and have all of your friends subscribe to it. When you load up the game to play it, make sure you are selecting the steam workshop version that you subscribed to and not the local version that's on your hard drive. The error your getting happens when you select the local version.

If you can't figure out which is which, or if you're only seeing one on your end, have your friend host instead so that you don't get confused between the two versions.