My friends can't join "unable connect to the world"

Hi, I have some problems to play with my fiends: when I am the host and they try to join to me or i invite them the game will not load and they will have an error "unable connect to the world" but when they invite me I can join and play without problems, we have already tried to delete and install again the game but nothing..i've called the xbox support and they can't help me cause it's aproblem in the game, how can i fix? (Sorry for my bad english)