BuddyPress.org

In fact you want to obtain on your maner something that BP does on his side. If i understand correctly, the whole community activity is already on the “activity” page. But if you need only somethings and not all, you can use filters.

Search in the doc for “filters” and how to implement them. What you see on blastr (witch is not a BP site) are latest posts essentialy and a big CSS work.