What do the numbers after an invitee's status in iCal mean?
These numbers are only present for users who have enabled the iCal "debug menu", using a third party configuration tweaking utility, or the command line

defaults write com.apple.iCal IncludeDebugMenu 1


which adds them, as part of an additional entry in each invitee's context menu, after his e-mail addresses:



The numbers themselves contain information about the invitee's version of 
MobileMe or iCloud.