iPhonus Sudoku Tournament offers free tournaments in Sudoku. You can join existing tournaments, create your own tournament (this is bonus feature and it’s use is limited), protect your tournament with password and then only invited users can join it.
Game itself is based on iPhonus Sudoku game and have same features.

Company:
http://www.iphonus.com/

App URL:
http://www.iphonus.com/sudokutournament/

This application is a web app, rather than one you buy in the iTunes Store. You may wish to read the following articles:

• What's a web app?
• How do I use a web app?