mechanics of apps like whatsup
<p>Hi I am trying to create an android app that would have the same kind of features as apps like whatsup, in the way that it let's you search your phone contacts, gmail or facebook contacts and invite them to use the app. So when two people are using it with each other they can exchange stuff by the internet wi-fi or blutooth.</p>

<p>My question is how do I go about doing that? Do I just access the phone contacts and message them by internet or sms and they can use it on the fly? If so, how can I know just by browsing my contacts that a friend uses the same app? If not, do I have to have a intermediate server doing all thejob of logging in and checking a contact if it already is registered in that server so we can use it together?</p>

<p>Thanks for any help.</p>
