At minimum, for most scenarios, two user roles need access to Inventory:
Optionally, another user role can be give visibility of the entire inventory without the ability to perform any actions within it.
This is all handled, like all aspects of the system, through role security.
Here is an example of the role permissions commonly granted to a site level role:
Anyone having anything to do with inventory, at the very least, will need to be granted access. Read here for definitions of each of the other permissions.