For the last couple of weeks I've not been able to join a fire team with my brother using the same network. However I'm quite able to join a random players fire team, is this an actual accident, or is this an insidious means of forcing players together? Who knows but the creators at Bungie! [spoiler]Wow such scorn much humour or is it?[/spoiler]
Try renaming each Console. My son and I were experiencing the same issue. His Xbox was Named Connor and mine was named Snake. I changed mine to SnakeXboxOne, and now we can play together with 0 issues. I hope this helps.