I can probably answer this, when you go to his main page it shows you how to setup a Raid sync setup for a leader and raid member.
Right now my guild is using this for raids, fp's and heroic runs for now.
right now, all of you have to be synced in the app inorder for everyone's combat log to be synced, thus showing everyone's damage. Currently, if they are not using the app, their damage will not populate the app.
It looks right now the combat log just shows your damage vs theirs, thus needing the sync to bridge the data not being recorded.