Serialize nested list object

I have a json like

{
   "meta":{
      "per":20,
      "page":1,
      "total":2
   },
   "users":[
      {
         "id":119506,
         "first_name":"erglk",
         "last_name":"wfe",
         "email":"bent@exemple.com",
         "groups":[
            {
               "id":5282,
               "name":"test"
            },
            {
               "id":8880,
               "name":"everybody"
            }
         ]
      },
      {
         "id":119507,
         "first_name":"eriglk",
         "last_name":"wife",
         "email":"benit@exemple.com",
         "groups":[
            {
               "id":5284,
               "name":"testf"
            },
            {
               "id":8880,
               "name":"everybody"
            }
         ]
      }
   ]
}

For the moment no problem to access to the user but I have some difficulties to access to the groups array. I've tried hasMany and belongsTo without success. I had errors. I've read few articles about EmbededRecordMixin but without any success.

If I declare in my models :

export default DS.Model.extend({
  first_name: DS.attr('string'),
  last_name: DS.attr('string'),
  email: DS.attr('string'),
  groups: DS.attr('group')
});

I get : Error while processing route: users Assertion Failed: Unable to find transform for 'group' Error: Assertion Failed: Unable to find transform for 'group'