Using Google Account to Log in to an Android Application

Android app login with Google Account

You might want to authenticate the user using one of the google account already configured in your device like some of the apps do, for that follow the below link -

"Authenticating to OAuth2 Services" - link
Download Sample from Google - Android SDK Manager/Extras/Google Play Services

In simple steps it does

Shows list of accounts in your mobile
Generates access token from selected accounts
Gets the Account name from access token by contacting google services(seperate call) to just tell that it has authenticated.
This is another link which is good in explaining the process link
you can follow below steps for Login in your app

you will send the generated access token to your Back-end server
Back-end server checks that access token is valid or not by contacting google services by this url

Next Back-end server responds to app whether to make the user login or not.
Below is response format of above "userinfo" call

you 'll get response as below

{
 "id": "ID",
 "name": "NAME",
 "given_name": "GiVEN NAME",
 "family_name": "FAMILY_NAME",
 "link": "https://plus.google.com/ID",
 "picture": "https://PHOTO.jpg",
 "gender": "GENDER",
 "locale": "LOCALE"
}

If you want Email id along with that response you have to modify

SCOPE = "oauth2:https://www.googleapis.com/auth/userinfo.profile";

to

SCOPE = "oauth2:https://www.googleapis.com/auth/userinfo.profile https://www.googleapis.com/auth/userinfo.email";

in that sample.

source

Android Application let specific users access it

You can do this easily using Firebase Authentication. Here is the documentation for it.
Just connect your app to Firebase, create login screen with Activity and create accounts with the usernames and passwords for your users and give them the credentials.

As for the Google Play billing policies, I believe this violates their rule that you should only get paid through Google Play so they get 30% of the income, however, if it all happens outside of the app, and there are no platforms to get paid through (like PayPal) integrated in the app itself, it shouldn't be a problem as far as I know.

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