Open the People Tab in the Folia Home to see everyone associated with the Folia. You can add people to your own Folia by tapping "Add a user" and entering their email address. Even if they don't have a Folia account, they will be invited to create one.
People you add to the Folia will be allowed to read your documents and use your annotations and comments. Change them from Readers to Coauthors to let them upload documents and edit your folia.