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.
This is how the sharing looks like in the parent BU:
This is what I'm supposed to see in the child BU, but am only able to see in the parent BU currently:
(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)