Why can't I add any more user?
You've reached the maximum numbers of users supported for your device. That's why the entry is grayed out. You can attempt to confirm this by executing the following command in a terminal emulator app or through adb:

pm get-max-users


If it shows Maximum supported users: 2 (Guest user is not included in maximum limit, so it doesn't matter whether it is setup or not; Primary user/Owner is included in the limit) then you know you hit the wall. The limit is set by the manufacturer.

Example for Nexus 6 running Android 5.0:

(Click an image to enlarge it)