Would be great to add a join in session in game. I have a group of friends playing destiny, Always see at least 8-12 min online playing every time I get home from work. Either they or I will be in a middle of something epic too busy to stop. Then would need to log into my friends list on Xbox One to do join, but the join game is not always available.
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You can by joining their fireteam. I do it all the time it just won't let you if their match is full otherwise you can. Note: I'm on ps4 doubt that this would make any difference but just keep this in mind
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It depends on where they are and what they are doing. I'm not sure exactly how it works, but I could join a friend when he was in the middle of the fight in the Black Garden, but not when he was on a standard mission. A few days later, vis versa happened. I think it's just another bug crawling around that might need to be fixed. Most of the time though, you should be able to join in, so long as the fire team is not full.