Why it's important to upload your app to Google Play before sharing it
If you've created an application that you plan to publish to Google Play, it is strongly recommended that you upload it to Google Play before you share the app with anyone else. Otherwise, you run the risk of having someone else publish your app to the market as their own creation, preventing you from publishing it yourself. While this may not often occur, it is something to be aware of.
Apps in Google Play are identified by their package name. Each package name can only be used by a single app; no two apps in the market can have the same package name.
When you start an Andromo project, a unique package name is generated for you. Once you upload your app to Google Play, its package name will forever be tied to your Google Play publisher account.
If you want to prevent someone else from taking your app and putting it on the market as their own, you need to put it on the market first, before you make it available to anyone.
If you did share your app publicly and someone else published it to Google Play before you did, you would be forced to create a new Andromo project (in order to generate a new package name) in order to publish your app on Google Play.
So, the best practice is to upload your .apk file to Google Play (even as a "draft" that isn't publicly available yet) before you give it to anyone else.