You might use the real time CloudDB. (or perhaps a FirebaseDB) Anyone who has the same apk can see changes made by other users. It is rather easy using Labels to display new information on all devices in which the app is actively runnint. It is significantly more difficult if a notification is required. (use an extension) as Tim has mentioned.
To start, here are a couple of links
There are other ways (google app inventor clouddb or firebasedb chat)
Since you are new , these esources to help you learn to use the AI2 tools A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook
http://www.appinventor.org/book2 ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
Regards,
Steve