I was able to get a basic multiplayer lobby up and running with ease - thanks to Microsoft's code samples. I quickly discovered however that my user interface, login code and animation storage all still assumed that a match will only contain 1 or 2 players. So I had to rewrite and redesign those areas, and I now have 4 player local multiplayer up and running. I also improved the aiming code and added the ability to select targets.
Next I need to convert the basic multiplayer lobby into one that will support 8 players on two teams. I would also like to allow more than one player to join an online match from the same xbox. Then we will see just how tricky implementing an online multiplayer game really is.