Currently, if one user in Pydio creates an address book entry for a contact with User ID "email@example.com" then another user that wants to setup a contact for the same client cannot use that User ID anymore. This create a situation where only one user can setup clients according to their email address and have this contact in their address book, and also means clients can end up having multiple logins depending on who’s sharing files with them. This really sux.
There needs to be some way for multiple users to have the same client in their address book0s. I would suggest that you allow users to create client records in their own address book, but provide a mechanism for other users to search across all address books in order to import a link to those client records into their own address books. If a user tries to create a client who’s email address is already used as UserID then the user should be allowed to link to that record instead of creating a new one.
As a suggestion, only the first user to create the client can administrate the record, but make it an option to create “shared” client records that can be edited by any user that links to it. Make that setting configurable to default on/off from an administrative config. Administrators will also need to be able to run reports on all users linking a client, and be able to administratively modify/delete these records.