Hi Blake,
Unfortunately as of right now, there is no direct way to set up screen level permission to allow users access to certain apps or screens. This is something we are working on implementing in the future however!
There are a few ways you could work around this in the meantime however. You could assign an action result OnScreenLoad which grabs the currently logged in User, and Show/Hide controls/fields on a screen based on this value. A bit of a hack solution I know, but it works. The downside of this would be that you would need to maintain a manual list of which users should be able to see which screens.
Theres other ways to tackle this one, but this would probably be the easiest to implement (and with the least redundancy)
Thanks Blake.
Ben