Thank you for your responses. That is a real shame.

Sadly, for some reason that nobody on the forums knows, Bungie did a stupid (I'm normally all for Bungie's changes and ideas, but this one really is a bit idiotic) and completely tied the fireteam restrictions (everyone/invite/disabled) to the Party Application on Xbox One. While, normally, that's fine and dandy, you get to play with your friends and it encourages using Xbox's Party Chat system, what it also does is makes it impossible to actually disable joining the fireteam from within the game; it seems Bungie forgot that they have programming in the game that lets you join your friends. Xbox One is the -only- system that suffers from this issue; Xbox 360 still had normal fireteam settings in-game, as does PS3 and PS4. The issue is that a friend of -any fireteam member- can join in-game as long as that fireteam member is visible on Xbox, because Destiny shows you all your friends online, and you can simply highlight them and join their fireteam. You don't have to be in the Xbox Party to join the fireteam, you can do so from within the game entirely, completely skipping any restrictions that the party has, such as invite only, etc. So, sadly, your -only- option is to request -everyone in your fireteam- to set themselves as invisible on Xbox One so their friends can't see them online, and thus not be able to join them. Or you can try to convince them to unfriend anyone that joins without permission or notification, since that's a rude thing to do.