Here's the backstory...
Are you grasping the fact that I am upset with the way salesforce.com has rolled out Communities as the end-all-be-all to partner and customer portal solutions?
To be fair, their Communities product offering is a much needed improvement to the 1990's facade that was the partner portal user interface (UI). Nonetheless, the new Communities UI is lacking a bit.
Although I could go much further with my rant regarding the shortcomings of the Communities product, I am going to focus on a small issue that I recently resolved but took forever (well maybe only a few days of troubleshooting) to resolve and the solution was so simple that I thought it required letting other Salesforce admins and developers in on the fix.
Here's the issue. Partner portal users are able to see all activities of other portal users yet communities users are not able to see all activities of other communities users. In my particular case Profiles and org-wide object sharing is such that Users with proper record access should be able to see Tasks & Events of other Users with access to the same records. However, without setting a boolean value on all activities to true, this is not the case.
Basically, there is a boolean field on Activities that drives visibility of activities to other Community Users. This fields is labeled "Public" (IsVisibileInSelfService) and is a standard field for both Tasks and Events. It turns out that unless you check this field the activities of Communities users are not visible to other Communities users even if you have the proper Profile and sharing settings in place. Odd? I think so.
Okay, so I jabbered on and on here with more BS than usual. The point I am trying to make is that if you are using Communities for more than Chatter and you are finding that Users are unable to see activities of other Users then the issue is that the IsVisibileInSelfService field is not set to true on your activities. Make sure to set that field to true and you will resolve your issue.
Salesforce Communities... WTF!