Coupons Website, Mrrebates.com Replica

Need a combination of two sites. We need a replica of mrrebates.com. Currently we own [login to view URL] and that will give you an idea of what the site should look like. Essentially, we are looking for simplicity of [login to view URL] (frontend) + the functionality of [login to view URL] (backend). Two additional functions are required: 1) Ability to register groups and allow registered users to join a group, 2) Ability to allow users to submit coupon codes from the respective store page. Need an efficient CMS for adding store/rebates/coupons pages (i.e. joomla). Logo is already created, any designwork can/will be provided. [login to view URL] already has some existing php for the functions. This can be integrated into the new site or programmer can use his/her own code.