Hi,
My name is Boris and I'm leading a small team of developers from Europe. We've been building Web apps for quite a while in .NET and PHP and can definitely jump into this one. Beside developers, we got one designer in house, who can easily slice PSDs and turn them into responsive Web apps.
I guess you're looking for PHP/MySQL based solution, so please let me know if you can talk yet about detailed architecture.
I'm available for Skype call, so let me know if around.
Cheers,
Boris