Sharing mobile app data with friends

[Richard Miller]

I'm looking for ideas on how to allow an app to share data between 
friends. For example, if someone has a picture on their phone they want 
to share with another, how can this be done elegantly and simply, 
directly from inside the app? Obviously, they can send it via SMS, but 
that takes place outside the app. Email can also be used, but that lacks 
elegance.

One option would be for the app to maintain a list of the user's 
friends, perhaps referenced by some phone-specific ID. The app can then 
send a picture to another by storing the pic on my server under the ID 
of the friend, which is then retrieved by the friend via the server. Is 
there any such built-in ID to work from, or do I need to assign ID's?

Any other ideas on how to accomplish this? I don't want to rely on some 
external shared app like Facebook or Twitter.

Thanks.
Richard Miller