How to create or signup user for the first time in firebase

Question

I am using firebase-auth and firebase-document elements in Polymer. I am signed in with "google" as provider. My firebase database is like below:

firebase-app-id-location
 [-]users
   [-]user1.uid
   [-]user2.uid

So i am planning to create new users as they sign up using "Google". Each of the node user1.uid will contain their own details. In order to keep everyones details private i set up the below security rules.

  "rules": {
    "users": {
      "$uid": {
        // grants write access to the owner of this user account whose uid must exactly match the key ($uid)
        ".write": "auth !== null && auth.uid === $uid",
        ".read": "auth !== null && auth.uid === $uid"

In order to test my code i have manually created a node under users using the firebase dashboard and everything is working fine. But NOW, How do user initiates the signup process? because the users/ node is only readable/writeable to the logged in user, how do I check if the user is already present under users/ and if not create a new new node users/user3.uid. In side firebase-document polymer element i am setting up the location as location = "firebase-app-location.com/users/user.uid". If the user is present then my app is working fine and fetching the data. But if the user is not present then i want to create a node under users/ and load some preset dummy data.

Any ideas? How to get this working.