A simple Google would have resulted in the API URLs from where you can get enough resources to get yourself started. Google Contacts API support .NET approach, you can get started from here:
https://developers.google.com/google-apps/contacts/v3/[
^].
Chances are that you want to "create contact", there is a section dedicated to this part, on how to create a new contact. Read that and learn how to authorize your application for updating the contact directory for a user and then how to update their directories and so on. I haven't tried it, but I guess, you are going to need knowledge of OAuth before using these services.
Read that entire page, and the ones related to it, you will be able to find out how to do that.