Re: multiple opponents / ending fights
You can't defend yourself against a group. You take care of one person then move on to the next. Perhaps you can borrow a video tape of multi-person attacks to give you a better idea.
If you need to pin in a multi-person attack, use an informal pin, i.e. where you are standing. In that way you can move quickly if attacked by another person. Formal pins from seiza are demonstrations of control. They are used in testing situations where you don't have to watch your back.
The way to end a fight is to have a non-fighting mind. In a real situation you're not out to prove anything. You only want to remain alive and hopefully not be injured. Walk or run away when you can.
There are also instructors like AikiWeb columnist George Ledyard, who have special classes and seminars dealing with multi-person attacks. If you're interested in the topic or expect to face this situation due to rank testing, you should consider taking a class.