Create a PFObject and PFRelation after PFUser Sign Up

In my structure, I would like to signUp a User and assign a Group to it directly (Many to many relationship model)

Below is how I signUp the user. after completion, I have no idea how to relate to the PFObject using PFRelation.

Any thoughts please?

// SIGN UP USER
var user = PFUser();
user.email = emailTextField.text;
user.username = emailTextField.text;
user.password = passwordTextField.text;

user.signUpInBackgroundWithBlock({ (succeeded: Bool, error: NSError?) -> Void in
    if error == nil {
        //Create a PFObject
        var group = CustomPFObject();
        group.name = "My First Group";
    }
});

You can do something like this:

user.signUpInBackgroundWithBlock({ (succeeded: Bool, error: NSError?) -> Void in
    if error == nil {

        //Create a PFObject

        var group = CustomPFObject();
        group.name = "My First Group";

        var relation: PFRelation = group.relationForKey("your_key")

        relation.addObject(user)

        group.save() // synchronous

        group.saveInBackgroundWithBlock { (Bool, NSError?) -> Void in

        }   // async
    }
});