It'll be a nice break from working on the calendar/event code, which isHeh. Books full of fiddly bits.
A couple of questions: as a moderator, do you want to see theseYes, I would definitely want them listed (and of course activity logged). Probably in the main Members list so I don't have to remember to check for them separately. Maybe with some kind of indication that they came about this way. And sortable (or searchable) so we can find them; perhaps a unique value for the Delivery column.
And what should we call these? Temporary subscriptions?Perhaps "instant subscriptions". It has a more positive connotation, and doesn't imply a time limit where there is none (they're limited to a thread, which may continue indefinitely). Or "reply-only subscriptions", which is descriptive (but a bit of a mouthful).
In a group with Restricted membership the "instant" member would require approval to convert to a regular membership.