I either join and kick a few of the players out of the fireteam or the game doesn't allow me to join and acts like I'm not attempting to join. Hard resetting the xbox worked, but when we disbanded the fireteam to change characters it happened again. I've checked my internet connection and nothing seems wrong or different than normal.
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Seems to be issues between last/next gen consoles.
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Just went through the same thing, my friend restarted the game on his end and it finally worked.