It's called AppShare(+) and you can get it here (requires root):
You can download apps on your main user account as usual, and then use AppShare to simply select which users you want to have access to the app.
This also works for "unsharing" (disabling) apps for users. For example, you could disable the Gmail, or Play Store apps for a 'Guest' user account.
As a bonus, you can also use the app to disable (but not uninstall) system or user apps on your main account, freeing up resources and potentially speeding up your device.
It was designed and optimised for tablets (tested on a Nexus 7), however it will also work on phones that have multi-user support enabled through a custom ROM.
Even though I am using the same method that the Android system uses to enable/disable apps, some system apps react in a weird way to being disabled and sometimes re-appear.
I have found that disabling certain system apps works best on a "fresh" account – i.e. A new user which hasn't had a Google account previously set up on it.
There are a couple more limitations, but I would suggest that you read the Play Store description for the full run down...
I have tested it as thoroughly as I can, but please let me know if you run into any problems.
I'd love to hear what you think and get some feedback.