To enable multi-user at the tab 3 10.1, first you musst root your stock rom.
Then instal BusyBox from the playstore open it and instal.
Now you take a build.prop editor (maybe from playstore) and open, now add a new property.
Name it: fw.max_users
At the key you fill how many users you want (2 at my tab)
Save and Reboot!
Now in the options, you shold can add new users.