Categories in a multi-user account plan have the same function as with the single user account plan. They help organize related items into a common group. In multi-user, categories are slightly different in that they have owners. An owner can be any one of your account members.
Anyone can create a category, but only the owner and the Administrator can delete or edit a category. Categories work in conjunction with Locations to help maintain the permissions for each user while allowing them to view groups of related data.
A Category will only be viewable in the Category list when an Item, (at least 1 Item), has been assigned to it. So in essence, the app sees no reason to display the Category in the list that is not linked to an Item. Once an Item has been added and the Category assigned by the Admin, then the Category will be shown in the list of Categories.
A user can only see categories that they either own, or contain items that they have permissions to view. Even if a user owns a category they can only see the items in Locations that they have permissions to see.