lets create a new activity for displaying group contacts

app/src/main/AndroidManifest.xml

@@ -74,6 +74,10 @@
             android:label="@string/settings"
             android:parentActivityName=".activities.MainActivity"/>
 
+        <activity
+            android:name=".activities.GroupContactsActivity"
+            android:parentActivityName=".activities.MainActivity"/>
+
         <activity
             android:name=".activities.ViewContactActivity"
             android:label="@string/details"

app/src/main/kotlin/com/simplemobiletools/contacts/activities/GroupContactsActivity.kt

@@ -0,0 +1,19 @@
+package com.simplemobiletools.contacts.activities
+
+import android.os.Bundle
+import com.simplemobiletools.commons.extensions.updateTextColors
+import com.simplemobiletools.contacts.R
+import com.simplemobiletools.contacts.helpers.GROUP
+import com.simplemobiletools.contacts.models.Group
+import kotlinx.android.synthetic.main.activity_group_contacts.*
+
+class GroupContactsActivity : SimpleActivity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_group_contacts)
+        updateTextColors(group_contacts_coordinator)
+
+        val group = intent.extras.getSerializable(GROUP) as Group
+        supportActionBar?.title = group.title
+    }
+}

app/src/main/kotlin/com/simplemobiletools/contacts/fragments/GroupsFragment.kt

@@ -1,24 +1,27 @@
 package com.simplemobiletools.contacts.fragments
 
 import android.content.Context
+import android.content.Intent
 import android.support.design.widget.CoordinatorLayout
 import android.util.AttributeSet
 import android.view.ViewGroup
 import com.simplemobiletools.commons.extensions.updateTextColors
+import com.simplemobiletools.contacts.activities.GroupContactsActivity
 import com.simplemobiletools.contacts.activities.MainActivity
 import com.simplemobiletools.contacts.activities.SimpleActivity
 import com.simplemobiletools.contacts.adapters.GroupsAdapter
 import com.simplemobiletools.contacts.dialogs.CreateNewGroupDialog
 import com.simplemobiletools.contacts.extensions.config
 import com.simplemobiletools.contacts.helpers.ContactsHelper
+import com.simplemobiletools.contacts.helpers.GROUP
 import com.simplemobiletools.contacts.interfaces.FragmentInterface
 import com.simplemobiletools.contacts.models.Contact
 import com.simplemobiletools.contacts.models.Group
 import kotlinx.android.synthetic.main.fragment_groups.view.*
 
 class GroupsFragment(context: Context, attributeSet: AttributeSet) : CoordinatorLayout(context, attributeSet), FragmentInterface {
-    var activity: MainActivity? = null
+    private var activity: MainActivity? = null
-    var lastContacts = ArrayList<Contact>()
+    private var lastContacts = ArrayList<Contact>()
 
     override fun setupFragment(activity: MainActivity) {
         if (this.activity == null) {
@@ -63,8 +66,10 @@ class GroupsFragment(context: Context, attributeSet: AttributeSet) : Coordinator
         val currAdapter = groups_list.adapter
         if (currAdapter == null) {
             GroupsAdapter(activity as SimpleActivity, storedGroups, groups_list, groups_fastscroller) {
-                val group = it as Group
+                Intent(activity, GroupContactsActivity::class.java).apply {
-                val groupContacts = contacts.filter { it.groups.map { it.id }.contains(group.id) }
+                    putExtra(GROUP, it as Group)
+                    activity!!.startActivity(this)
+                }
             }.apply {
                 setupDragListener(true)
                 addVerticalDividers(true)

app/src/main/kotlin/com/simplemobiletools/contacts/helpers/Constants.kt

@@ -13,6 +13,7 @@ const val ON_CONTACT_CLICK = "on_contact_click"
 const val CONTACT_ID = "contact_id"
 const val SMT_PRIVATE = "smt_private"   // used at the contact source of local contacts hidden from other apps
 const val IS_PRIVATE = "is_private"
+const val GROUP = "group"
 
 // contact photo changes
 const val PHOTO_ADDED = 1

app/src/main/kotlin/com/simplemobiletools/contacts/models/Group.kt

@@ -1,6 +1,12 @@
 package com.simplemobiletools.contacts.models
 
-data class Group(var id: Long, var title: String, var contactsCount: Int = 0) {
+import java.io.Serializable
+
+data class Group(var id: Long, var title: String, var contactsCount: Int = 0) : Serializable {
+    companion object {
+        private const val serialVersionUID = -1384515348451345L
+    }
+
     fun addContact() = contactsCount++
 
     fun getBubbleText() = title

app/src/main/res/layout/activity_group_contacts.xml

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="utf-8"?>
+<android.support.design.widget.CoordinatorLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    android:id="@+id/group_contacts_coordinator"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent">
+
+    <RelativeLayout
+        android:id="@+id/group_contacts_wrapper"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content">
+
+        <com.simplemobiletools.commons.views.MyRecyclerView
+            android:id="@+id/group_contacts_list"
+            android:layout_width="match_parent"
+            android:layout_height="match_parent"
+            android:clipToPadding="false"
+            android:scrollbars="none"
+            app:layoutManager="com.simplemobiletools.commons.views.MyLinearLayoutManager"/>
+
+        <com.simplemobiletools.commons.views.FastScroller
+            android:id="@+id/group_contacts_fastscroller"
+            android:layout_width="wrap_content"
+            android:layout_height="match_parent"
+            android:layout_alignParentEnd="true"
+            android:layout_alignParentRight="true"
+            android:paddingLeft="@dimen/normal_margin"
+            android:paddingStart="@dimen/normal_margin">
+
+            <include layout="@layout/fastscroller_handle_vertical"/>
+
+        </com.simplemobiletools.commons.views.FastScroller>
+    </RelativeLayout>
+
+    <com.simplemobiletools.commons.views.MyFloatingActionButton
+        android:id="@+id/group_contacts_fab"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_gravity="bottom|end"
+        android:layout_margin="@dimen/activity_margin"
+        android:src="@drawable/ic_plus"/>
+
+</android.support.design.widget.CoordinatorLayout>