basically
its, when sum1 joins your event or posts in it,
some sort of a notification telling you.
when u dont get any in your event for a while u dont bother checking
and when u check u see sum1 joined and wants ls 2 days later
its a easy simple add,
which u can choose maybe to have switched on or off when creating your event,.

I highly doubt this will be implimented. Paul incorporated something like this into the Jsp IM update already.

http://forums.d2jsp.org/topic.php?t=41813019&f=28

[x] Pointless

DIM already has this feature

events system is a piece of shit and this wont make it better

nothing will

Check the Site Announcement out from last month:
http://forums.d2jsp.org/topic.php?t=41813019&f=28&v=1

There already are INSTANT notifications of ALL those occurances via the dIM.