table & model changes for group managers with permission to edit membership

db/migrate/20150108221703_group_managers.rb

@@ -0,0 +1,11 @@
+class GroupManagers < ActiveRecord::Migration
+  def change
+    create_table :group_managers do |t|
+      t.integer :group_id, null: false
+      t.integer :user_id, null: false
+      t.timestamps
+    end
+
+    add_index :group_managers, [:group_id, :user_id], unique: true
+  end
+end