In MediaValet, you can decide which categories your users have access to by using Custom Groups!
Here is where you can manage the Category access:
Go to Settings > Groups, then under the Access column click on View/ Edit
From there, you just need to select the categories you want this user group to have access to:
Then click Save when you're done!
Please be aware that you can give access to a maximum of 50 categories. You can see the count at the bottom of the window.