Can you only share assets or folders that you are the owner of? It is this way in our environment and I am not sure if this is by design or a configuration issue. Seems counter intuitive to have so many Owners of folders to make this feature work.
Review here for more details - https://docs.adobe.com/docs/en/aem/6-2/administer/content/assets/link-sharing.html. Ideally in any system you would only see the content on which you have access to. If you have tons of users of same ACLs you would put them in common group and add group to rights mgmt in folder or hierarchy.