The old TiVO ToGo feature is no longer officially supported and the ability to turn on sharing seems to have been removed from the My Account page on the TiVO Account management site.
But at least for TiVO Series 2 and Series 3 HD, it still does work.
One curious bit of information, it seems if a TiVO is Reset using Guided Setup the sharing TiVOs "must" be in the same ZipCode.
If they are not, it appears this is one measure for them not being in the same household and sharing will not work. TiVOs reset with the guided setup and in the same Zip Code can "share and see one another" but TiVOs setup in different Zip Codes cannot see one another even if on the same local LAN in the same household.
TiVO Desktop for transferring recording to a desktop, also works between PC and Mac with the proper software installed, and licensed for the PC or Mac.
Presumably this also means the same thing for PyTivo.
UPDATE: different day, different Tivo HD
The New TivoHD had to get added to my account, then it had to update its MAC Key to mine by contacting Tivo.. but the other Tivos on my subnet could see but not share videos, and the New Tivo could see them but not see videos on them. Turned out the other Tivos had to contact Tivo and then they could see each others videos. I initiated a Tivo network update, and only after it went though all but the last step of loading guide data.. di they start to see each others recordings.
So its not just that the New Tivo needs to contact Tivo and update its MAC key.. and that it have Tivo sharing turned on.. but also needs for the other Tivos.. even if they have the exact same MAC key.. both sides need to contact Tivo and then sharing starts working.
I wonder if the MAC key is more a pointer to a table or permission list held at Tivo.. that downloads a list of Tivos for the end user Tivos that says its okay to share recordings between them. So basically.. when they contact Tivo Central.. they download the updated recording share lists for your subnet.
It also seems to be mac address based, since I had a burned out ethernet port on one Tivo that never shares.. but it seems the Tivo share list won't update to include it.
Ironic is the "MAC" key refers to a user table like a hosts or DNS list of Tivos on the local subnet that are allowed to share. They can "see" each other by MDNS or some type of Bonjour.. but don't have permissions until the updated MAC list is downloaded into each Tivo.