I am trying to share with my son on the same computer, an iMac. Under both steam accounts, the computer is authorized. Both accounts are authorized (he is authorized under my account, I am authorized under his account).
Under his iMac profile, running steam and looking in the games library, nothing looks any different. There is no sharing going on.
But under my iMac profile using his steam credentials, I do see games as being shared. However, the whole point is for him to use his iMac profile and his Steam credentials.