Add GroupsService to fetch data from server

app/assets/javascripts/groups/index.js

+/* eslint-disable no-unused-vars */
+
 import Vue from 'vue';
 import GroupsStore from './stores/groups_store';
+import GroupsService from './services/groups_service';
 
 $(() => {
+  const appEl = document.querySelector('.js-groups-list-holder');
+
   const groupsStore = new GroupsStore();
+  const groupsService = new GroupsService(appEl.dataset.endpoint);
 
   const GroupsApp = new Vue({
-    el: document.querySelector('.js-groups-list-holder'),
+    el: appEl,
     data: groupsStore,
+    mounted() {
+      groupsService.getGroups()
+        .then((response) => {
+          this.groups = response.json();
+        })
+        .catch(() => {
+          // TODO: Handle error
+        });
+    },
   });
 });

app/assets/javascripts/groups/services/groups_service.js

+import Vue from 'vue';
+import VueResource from 'vue-resource';
+
+Vue.use(VueResource);
+
+export default class GroupsService {
+  constructor(endpoint) {
+    this.groups = Vue.resource(endpoint);
+  }
+
+  getGroups() {
+    return this.groups.get();
+  }
+}

app/views/dashboard/groups/_groups.html.haml

-.js-groups-list-holder
+.js-groups-list-holder{ data: { endpoint: dashboard_groups_path(format: :json) } }
   %ul.content-list
     - @groups.each do |group|
       - group_member = group.group_members.find_by(user_id: current_user)