Way to auto-group existing photos into events?
In iPhoto > Preferences > General there is an option "Autosplit into Events" that you can set. 

If all you are seeing is the one Photostream event, I'd guess that it isn't finished importing and it will split the events when it is done.

I don't have a photostream event in my events in iPhoto, I just have an iCloud tab that is on the same level in the sidebar as Events.