https://discuss.emberjs.com/t/how-do-i-configure-a-many-to-many-relationship/15158

mirage.config.js

export default function() {
  this.resource('users');
  this.resource('accounts');
}

mirage.models.account.js

import { Model, hasMany } from 'ember-cli-mirage';

export default Model.extend({
  users: hasMany()
});

mirage.models.user.js

import { Model, hasMany } from 'ember-cli-mirage';

export default Model.extend({
  accounts: hasMany
});

mirage.serializers.application.js

import { JSONAPISerializer } from 'ember-cli-mirage';

export default JSONAPISerializer.extend({
});

models.account.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  users: hasMany()
});

models.user.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  accounts: hasMany()
});

router.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('sign-up');
});

export default Router;

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return this.store.findAll('account');
  }
});

routes.sign-up.js

import Ember from 'ember';

export default Ember.Route.extend({
  model () {
    let account = this.store.createRecord('account');
    let dentist = this.store.createRecord('user');
    let officeContact = this.store.createRecord('user');

    // store.createRecord is not async, therefore returning POJO is fine
    return {
        dentist: dentist,
        officeContact: officeContact,
        account: account,
    };
},

actions: {
    register () {
        let item = this.modelFor(this.routeName);
      
        item.account.save()
            .then(function() {
                item.account.get('users').addObject(item.dentist);
                item.account.get('users').addObject(item.officeContact);

                item.dentist.save().then(function() {
                    item.officeContact.save()
                })
            }).then(() => this.transitionTo('/'))
    }
}
});

templates.application.hbs

<p><i>Reproduction for https://discuss.emberjs.com/t/how-do-i-configure-a-many-to-many-relationship/15158</i></p>

{{#link-to 'sign-up'}}sign up{{/link-to}}

{{outlet}}

templates.index.hbs

<h1>Accounts</h1>

{{#each model as |account|}}
  <h1>{{account.name}}</h1>
  <ul>
  {{#each account.users as |user|}}
  	<li>{{user.name}}</li>
  {{/each}}
  </ul>
{{/each}}

templates.sign-up.hbs

<h1>sign up</h1>
Account name: <input type="text" onchange={{action (mut model.account.name) value="target.value"}}><br>
Dentist name: <input type="text" onchange={{action (mut model.dentist.name) value="target.value"}}><br>
Office contact name: <input type="text" onchange={{action (mut model.officeContact.name) value="target.value"}}><br>
<button onclick={{route-action 'register'}}>sign up</button>

twiddle.json

{
  "version": "0.15.0",
  "ENV": {
    "ember-cli-mirage": {
    	"enabled": true
		}
	},
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0",
    "ember-cli-mirage": "0.4.7",
    "ember-route-action-helper": "2.0.6"
  }
}