Do you ninja join? Which is to say join your friends' fireteam uninvited.
I do. When doing "Kill X boss" bounties it helps to take advantage of a friend's progress to bypass part of the associated mission. I encourage them to do the same to me as well. There is really no downside to have extra people in your fireteam.
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I haven't done it but I don't mind when it happens to me. I get the same reward no matter what, and completing the last section will be easier.