Remember if you want to HOST there's 2 options:
ADVERTISE or NOT TO ADVERTISE:
NOT TO ADVERTISE and TO ADVERTISE still has the same condition: Port Forwarded.
Try finding any programs or modify your router's Port ( Start and End ) to make the Port Forwarded Correctly to the MS...
right, back to the story:
ADVERTISE ( as you might know ) : If you forward your serv. correctly, it will show your server on the list for everyone to join.
NOT TO ADVERTISE : If your server is forwarded correctly, no one can see you in the JOIN GAME(INTERNET) feature, but can join by putting your IP in JOIN GAME(IP ADDRESS)
To join, you don't have to forward it correctly, but you must have a connection ( well, duh... )