I first want to say I really think the Xbox APP is amazing allowing you to communicate in parties directly from your smartphone. I've been using the App to communicate in-game because my cat decided to chew through my Astro headset wires. While I await for my replacement wires to arrive, i've noticed that I'm unable to join certain parties, especially from the "Looking for Group" feature.
I am unable to send party requests, or join party requests from others. I have no issues sending a party invite when I'm not the host. I've received a notification when trying to join a party invite from someone stating that I'm "unable to join party." This sucks because it seems I cant play with anyone who is not on my friends list.
I'm not sure if this a bug or not, but I figured i'd bring it up.