client.js

var confirmationQueue = new Firebase("https://static-showdown.firebaseio.com/email_confirmations");

imaginaryRouter.route("/confirm_email/:code", function(params) {
  confirmationQueue.child(params.code).remove();
});

rules.json

{"rules":{
  "email_confirmations": {
    ".write": "root.child('users').child(auth.userId).child('confirmed').val() == true"
  }
}}

server.js

var mail = require("nodemailer").mail;
var Firebase = require("firebase");
var base = new Firebase("https://static-showdown.firebaseio.com/");
var mailQueue = base.child("group_invitations");
var confirmationQueue = base.child("email_confirmations");
var users = base.child("users");

mailQueue.on('child_added', function (snapshot) {
  confirmationQueue.push({email: snapshot.child('invited_party').val()}, function (snapshot) {
    mail({
      from: snapshot.child('inviting_party').val(), // sender address
      to: snapshot.child('invited_party').val(), // list of receivers
      subject: "Join my StaticShowdown Crew", // Subject line
      text: "https://staticshowdown.com/#/confirm_email/" + snapshot.name()
    }, (error, response) {
      if (error) raise "handle error"
      mailQueue.child(snapshot.name()).remove()
    });
  })
});

confirmationQueue.on('child_removed', function (snapshot) {
  var email = snapshot.child('email').val();
  users.child(email).child('confirmed').set(true);
})