https://stackoverflow.com/questions/44232823/ember-multiselect-onchange

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  init()
  {
    this._super(...arguments);
    this.set('options',['one','two','three']);
    this.set('selectedOptions',[]);
  },
  actions:{
    updateSelection(event){
      let selctedValue = event.target.value;
      if(!this.get('selectedOptions').includes(selctedValue)){
        this.get('selectedOptions').pushObject(selctedValue);
      }
    }
  }
  
});

helpers.is-available.js

import Ember from 'ember';

export function isAvailable([opt,selectedOptions]/*, hash*/) {
  console.log(' opt ',opt,'selectedOptions',selectedOptions );  
  return selectedOptions.includes(opt);
}

export default Ember.Helper.helper(isAvailable);

templates.application.hbs

<select onchange={{action "updateSelection"}} multiple>
{{#each options as |opt|}}
 <option selected={{isAvailable opt selectedOptions}}>{{opt}}</option>
{{/each}}
</select>

Selected Options:
{{#each selectedOptions as |opt|}}
 {{opt}}
{{/each}}

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}