Are you inviting all at the same time? If you invite one by one that sometimes helps. Additionally whoever you are having a problem with get the host to go back to the chat section, ask the person to leave in room chat group, then re invite that person back into the chat. go bask to the GAME setup Lobby and if he does not come back in automatically re invite him and ask him to press the small button next to the left joystick one invite comes through.
I hope that helps.
Allan
