After inviting users and possibly creating user groups, you have the possibility to share files or folders with them. Just right-click on it, or go to the File Actions menu after selecting it, and choose "Share".
Unlike a Link whose purpose is to share a file with someone outside your contacts, a "Share" is intended to work collaboratively with your contacts.
A popup window allows you to define with whom these files will be shared.
It can be a single user.
Or an entire group.
Shared files have a specific icon next to the date.