Recently I changed the Microsoft account that my Xbox 360 profile is linked to (figured it might be relevant to the situation). Originally, it was on my dad's (since he was the one who bought it), but after some work I managed to transfer it over to mine. I was able to confirm this using the Xbox app on my computer which runs Windows 10, and it's through that app that I've been able to add friends from this group. It further confirmed this by showing those people on my friend list for this site. However when I checked the friends list on my Xbox, none of them were added. I'm not sure if this is because the app itself isn't connected to my Xbox, or if it's something else, but I think it'll be a real problem when I want to join group events and I can't connect to anyone properly.