Based on URL e.g.
https://plus.google.com/123456789012345678901 # dummy number
How can I find my gmail username?
If you are developing an application or script, you can use Oauth 2.0 and request the userinfo.email scope.
See https://stackoverflow.com/a/166778/ for more information.
This requires that you configure the Google API Console project and create an OAuth 2.0 client identifier.
You can not. For privacy reasons, having someone in your circle or knowing their Google Plus ID should not be sufficient to get an email address.
Unable to find a user’s arbitrary email address solely from the identifier contained in this Google+ .