IU has implemented a feature in Roster that displays a log of when users were added/removed from a site. The information is stored in a new database table. The page will display the following information (which will also be sortable):
Event (Add or Removed)
Source (What user did the action or how did a user join/leave a site, e.g. Membership tool, external automated source)
The screenshot is what the page looks like in IU's implementation of Sakai and certainly won't be the final product for Sakai.
I do need to do a bit of refactoring to make it more friendly to generic.