I think it's due to the NAT setting your router. In basic terms, you can have 3 types of setting: open, moderate or strict. Open can join anyone, moderate can join open and moderate, and strict can only join open. There are ways to change your NAT to open by tinkering with your router settings.
I will be back on Halo tomorrow night, I think we should organise a big NE sesh for around then? Hopefully we can get a party of 8 going and then we can boss big team battle.