Multi-user support is one of the most interesting additions in Jelly Bean 4.2, but you can only get it if you're using a tablet. It makes sense - phones are rarely shared between more than one person, while tablets are naturally shareable. Even so, it would be nice if Google gave users the option. But thanks to modder extraordinaire Paul "Modaco" O'Brien, there's a relatively easy way to enable multi-user mode on smartphones. It's detailed on the Modaco blog, but all you need to get started is a phone running Android 4.2 and root permissions.
First, download and install the Xposed Framework, a Swiss army knife for modders and the basis of this modification. Next, download the Modaco Toolkit, a module for the Xposed app. Open Xposed and tap "install/update" to enable all of its mod goodies. Then tap the Modules pane, enable the Modaco Tooklit, and tap "Google Nexus (all)". Finally, enable "Show users on power menu" and reboot your phone. If all goes well, you should see the Users option in your phone's Settings menu, enabling the multi-user mode seen on the Nexus 7 and Nexus 10.
Paul O'Brien tested this method out on his Nexus 4 (probably running Modaco's flavor of Android 4.2) and I tried it on my own Galaxy Nexus, running an AOKP build that's a few weeks behind. In both cases, the mod worked. That leads me to believe that it should function on most AOSP-based ROMs. Whether it will work on other 4.2 phones like the Galaxy S4 or HTC One remains to be seen - all the technical elements are there, but manufacturer skins have a way of gumming up the works. Are there any root users out there willing to give it a try? If so, head on over to Modaco's site and give some feedback, if you'd be so kind.