I just thought about this some more and had an “ah ha”.
Basically — if you make these “shared accounts” fully work as first class regular accounts, then they will automatically work with any app, in which case using the PDS service entry in the DID doc makes sense.
The flow would look like this
Let’s say I want to run a shared calendar on smokesignal.events
I create an SDS account, give myself @ngerakines.me @knowtheory.net @tynanpurdy.com access to it. I use the domain handle calendar.atprotocol.community
On smoke signal, I put in that handle. It bounces to the SDS server for auth. I then auth in with my bmann.ca account, and I return to smoke signal logged in with this calendar account.
This would be huge for ANY ATProto app!