Here are the options:

1. One is between the Ritz Carlton and the Diplomatic Club.

2. The other is south of the Intercon Hotel. You will get there from behind Qatar Exhibition Center. The Intercon will be to your left.

The third option (though I think it's for for families only) is Qtel Oasis in Sealine. They do have play ground setup for volleyball. They are open on Fridays from 14:00-22:00 but again, it could be for families only. (Do we have any girls here?). Here is more about the Oasis: http://www.qtel.com.qa/NewsFull.do?News=13721

Sam