My thinking on this is that we literally just send the user a notification, which shows up in the notification menu:

+-----+
| /'\ |
+-----+-----------------------------+
| epriestley mentioned you in Z123. |
|-----------------------------------|
| chad created D12345.              |
|-----------------------------------|
| epriestley closed T456.           |
+-----------------------------------+

Technically, this might be involved:

• It may be hard to get working with feed so that it doesn't show a feed story?
• It should take you to the message (not just to /Z123), but that's sort of hard, maybe?
• It shouldn't ever send you email, probably?
• It would be nice to include the message text but that might be messy.

Special casing that stuff might be enough work that it's not worth dealing with.

Specifically, I'd expect these unusual behaviors from these notifications:

• They do not generate feed stories.
• They do not generate email.
• They take you directly to the message when clicked, not to the thread.
• They should probably aggregate, ideally?

Maybe it's even worth separating these out as some kind of purpose-built thing in Conpherence, or just dropping the idea completely for now or forever. It seems nice to be able to summon someone to a thread by mentioning them, but cheaper approaches are:

• Make mentioning users who can see a thread add them, like how other objects work?
• Just add them as a participant if you want them to see stuff?

This seems like the least useful leg of T7577 to me.