We have made multiplayer flash games before. I am sure we can make your games easily too. We will custom make your game to your specifications.
One thing though, you will need to run a multiplayer game server on the server side (Only for Multiplayer Games). It is free, but will either need a VPS or Dedicated server. We can always run one for you too.
Game can easily support over a thousand users, will be secure with login and all the features you need.
To see some of the flash games we have made, visit bigleapgames