If you feel you need to report someone and cannot do so from within the game but they were in an activity you took part in:
Please go to your Bungie.net profile and view your game history.
Locate the match/activity report where this person was sabotaging your gameplay.
From there, you can report them equivalent to an in-game report.
Please do not go to their Bungie.net profile, as that just reports for conduct on the Bungie.net website and forums, and will not work for reporting in-game activity.