sudokucup.com

I've some good news for you!
Right now you can send an e-mail challenge after you successfully finish the practice puzzle.

It's not finished yet but you'll see a link which you can copy and send via email/PM/whatever to invite your friends to try to beat your time!
If you'll have any problem please let me know!

http://sudokucup.com/node/2588

Enjoy!