The latest version allows you to specify email address in GoogleApi.GetContacts method:
GoogleApi google = new GoogleApi(accessToken);
XmlDocument document = google.GetContacts(userEmail);
Remember to authorize both scopes (https://www.google.com/m8/feeds/, https://mail.google.com/) at:
https://www.google.com/a/cpanel/[your domain]/ManageOauthClients
AssertionFlowClient provider = new AssertionFlowClient(server, certificate)
{
ServiceAccountId = serviceAccountEmail,
Scope = string.Join(" ", new[] {GoogleScope.ImapAndSmtp.Name, GoogeScope.ContactsScope.Name}),
ServiceAccountUser = userEmail,
};