Android : Send data (communicating) from Google App Engine to 1: Android and iOS phones, 2: PCs

I am trying to develop 1. Apps for Android and iOS devices (customers and friends of customers who have my app on their phone) 2. Software that runs on PCs (store vendors)

I want to develop a Google App Engine application to manage interaction between my mobile apps and the PC software.

Scenario: 1. Mobile user A posts something from the device to App Engine 2. App Engine processes the data sent from the mobile user. 3. Based on the request from user A, App Engine sends data and communicates with: a) Specific mobile users (say, friends of user A) b) Specific PCs running my software (store vendors catering specifically to user A's request)

I want to know how to accomplish 3a and 3b. Please let me know what I have to do in: 1. App Engine application 2. PC software 3. Mobile app

Thanks.