|So your friends can join eachother but its only you that cannot join?|
1) check your xbox settings to ensure you have not blocked any of your friends
2) Check you NAT settings, to do this go into settings (xbox) then go to system Settings, Network Settings, Wireless network, Test xbox live connection.
this will either pass or fail, but if there are any restrictions on your network it will flag them up.
You may need to open a port in your router for the xbox.
other than that all i can say is that black ops and black ops 2 are the worst games i have ever played for match making, i have continous issues playing online with my friends too, sometimes for no reason it kicks us out of the party, throws one or two players in other lobbies etc.
anyways good luck