In the parent Business Unit, a folder in Content Builder is shared with a couple of child BUs. As far as I can see this, everything has been set up according to the documentation. However, in the Child-BUs the "Shared-Tab" isn't visible in the Content Builder interface.
The user I've tested this with has full administrator rights (Administrator and Marketing Cloud Administrator roles). Furthermore, I checked if any individual permissions have been set or anything else that could lead to not having the correct permissions. Everything seems right tough.
(Source of images: https://success.salesforce.com/answers?id=9063A000000iV6zQAE; it looks the same in my instance, but I didn't want to disclose production information)