How can I get the user's email address if I have the access token of that user in my google plus apps?
Getting the user's email address requires additional authorization. You will need to add the https://www.googleapis.com/auth/userinfo.email scope and can then use the userinfo endpoint to request the information. See https://developers.google.com/accounts/docs/OAuth2 for more information.
perhaps it helps https://developers.google.com/+/api/latest/people#resource