I would like to answer this question of yours. There are many methods to this, you can use your own server (or a website) to let different users to communicate with each other and share their files, or you can let them directly communicate. In your case you want them to directly send the files to other mobile devices. You can consider using the Sockets, to send the data through a device using a Windows OS with .NET framework.
Working with Sockets in C#[
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http://msdn.microsoft.com/en-us/library/system.net.sockets.socket(v=vs.110).aspx[
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The main technique involved in this, is the bluetooth support. If the device supports a bluetooth protocol then you can send the files via bluetooth. In Windows, you can perform these actions and send the files over bluetooth to any device that can communicate via bluetooth.
Read these documents[
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My choice: I would like to use the web server method, to create a server from where the users would share their resources, because bluetooth is only available if the devices are close to each other.