EDIT: Gareth’s advice is good
Alternatively you will probably need some kind of membership plugin off CodeCanyon to do this. You would need to check with the author on theme compatibility
Something to be aware of: if you are charging people to use the site e.g. the membership you require an extended license for any THEME which you use NOT the regular one http://themeforest.net/licenses/faq#license-paying-users-a
I would advise you to submit it to ThemeForest. Once it’s listed for sale, your item is immediately ‘protected’ from copyright infringements by others since there’s a proof that it’s your work. Also, if it’s great, some authors might approach you to convert it to a WordPress theme, and that’s when you can negotiate your deal.
This is the best advice possible. Get the design completed and submitted. Approved for PSD will help massively in finding the best developers plus will give you a better understanding of your potential sales