gistfile1.js

var MembersNewRoute = Ember.Route.extend({
  email: '',

  beforeModel: function(transition) {
    var registrantKey = transition.params.registrant_key;

    this.store.find('registrant', registrantKey).then(function(registrant) {
      // "this" is undefined
      this.set('email', registrant.email);
    });
    
  },

  model: function(params) {
    return this.store.createRecord('member', {
      registrationKey: params.registrant_key,
      email: this.get('email')
    });
  },

});

export default MembersNewRoute;