I often get messages from other players asking me to join them but I can't seem to make the connection. When I find the players character who issued the invite in my roster and try to select "join" I get a message telling me that acceptance is limited to friends only. I feel like an idiot as my avatar stands next to a group in the tower and I struggle to find the proper procedure which will allow me to accept their leaders invitation to join them. I'm on xbox 1.
It's possible they have their game/fireteam set to "Require Invites", which would stop you from joining from the game roster. If they have sent you an invite through Xbox, and you missed the notification that popped up, you can get back to those by going to the xb1 home/dashboard screen, and the little square icon in the upper-left corner has your recent xbox notifications.
They could. Be higher level than you ??
Add each other then. Seems like a simple solution. And on the XB1, you can have a bunch of friends.