Have you worked out how they are going to find each other and how they are going to connect to each other?
Will they need fixed ip addresses and the ability to accept incoming connection requests?
Some mobile carriers restrict incoming traffic if not in response. This stops you from having an 'open' server on their mobile network.
Data. Do you understand the data use this would be per user. If everyrhing they store is on their own phones anytime ANYONE accessed it, it would need to be sent to each request. Ie: a 5mb picture sent to 200 friends just ate 1G of data.
This all seems highly impractical. There are secure ways of storing data. However, as in the case of LavaBit, if you allow US based access, and they find your service is being used for illegal activities, they will shut you down. Most countries cooperate with this. Google 'lavabit'.
I am a system admin for a hosting company, which I am partner in ownership. I am highly familiar with web technologies. Under the constraints of mobile carriers, dns impossibilities without a server, I think you will find your app very very hard to bring to fruition.
Your app would be so slow no one would even consider using it.
Even if you restricted to WiFi only, the average home provider is 100 mbps, some 300 mbps. This is normally DL speeds, and uploads run 25 mbps, 75 mbps respectively. (These are averages).
This is why it is next to impossible to run a webserver from your home. Also, ALMOST ALL make this illegal in their terms of use.
IF you wanted it mobile, the upload on average 3G would make your app crawl. And, logistically, if you could connect 200 friends to one phone, the upload of a picture would CRAWL.
:)
But with some central hardware, the idea of keeping your more sensative data local and only shared securely (by approval case by case) albeit somewhat delayed (waiting for your aporovals and your device to poll for and you to respond to stored requests) may have merit if you really can make it all more secure than a properly run public facing server.
This is not freelancer.com. This forum is for DroidScript. You need to hire a programmer, purchase servers.
Neither are DS related.