You both need to be logged into an on-line arena, or connected by a LAN using 8 Player. Open the Roster. Right click your mouse on the person you would like to join. Select Join.
The person you selected will get a message asking to accept or deny the request to join. He needs to accept.
If he is in a bomber, you will find youself in his plane. Move the the gunner positions by using the 1 to 8 keys (depending on available positions).
Gunner