diff --git a/app/models/group.rb b/app/models/group.rb
index 8962bc83..060c5b70 100644
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -1,4 +1,12 @@
 class Group < ActiveRecord::Base
   belongs_to :user
-  has_many :users
+  has_many :group_members, dependent: :destroy
+
+  before_validation do
+    self.name = self.display_name.downcase.sub(/\s+/, '-')
+  end
+
+  def add_member(user)
+    GroupMember.create! group: self, user: user
+  end
 end
diff --git a/app/models/group_member.rb b/app/models/group_member.rb
new file mode 100644
index 00000000..3cd7913b
--- /dev/null
+++ b/app/models/group_member.rb
@@ -0,0 +1,4 @@
+class GroupMember < ActiveRecord::Base
+  has_one :user
+  has_one :group
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index dd0bb853..931feb6e 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -26,7 +26,7 @@ class User < ActiveRecord::Base
   has_many :moderation_comments, dependent: :destroy
   has_many :moderation_votes, dependent: :destroy
   has_many :groups, dependent: :destroy
-  belongs_to :group, foreign_key: :target_id
+  has_many :group_memberships, class_name: "GroupMember", dependent: :destroy
 
   SCREEN_NAME_REGEX = /\A[a-zA-Z0-9_]{1,16}\z/
   WEBSITE_REGEX = /https?:\/\/([A-Za-z.\-]+)\/?(?:.*)/i
diff --git a/db/migrate/20150108121010_create_groups.rb b/db/migrate/20150108121010_create_groups.rb
index a66886bd..252ae75b 100644
--- a/db/migrate/20150108121010_create_groups.rb
+++ b/db/migrate/20150108121010_create_groups.rb
@@ -2,16 +2,26 @@ class CreateGroups < ActiveRecord::Migration
   def change
     create_table :groups do |t|
       t.integer :user_id, null: false
-      t.integer :target_id, null: false
       t.string :name
       t.string :display_name
+      t.boolean :private, default: true
 
       t.timestamps
     end
 
     add_index :groups, :user_id
-    add_index :groups, :target_id
     add_index :groups, :name
     add_index :groups, [:user_id, :name], unique: true
+
+    create_table :group_members do |t|
+      t.integer :group_id, null: false
+      t.integer :user_id, null: false
+
+      t.timestamps
+    end
+
+    add_index :group_members, :group_id
+    add_index :group_members, :user_id
+    add_index :group_members, [:group_id, :user_id], unique: true
   end
 end
diff --git a/db/schema.rb b/db/schema.rb
index 860473f8..4a23f1b5 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -38,17 +38,27 @@ ActiveRecord::Schema.define(version: 20150108121010) do
 
   add_index "comments", ["user_id", "created_at"], name: "index_comments_on_user_id_and_created_at", using: :btree
 
+  create_table "group_members", force: true do |t|
+    t.integer  "group_id",   null: false
+    t.integer  "user_id",    null: false
+    t.datetime "created_at"
+    t.datetime "updated_at"
+  end
+
+  add_index "group_members", ["group_id", "user_id"], name: "index_group_members_on_group_id_and_user_id", unique: true, using: :btree
+  add_index "group_members", ["group_id"], name: "index_group_members_on_group_id", using: :btree
+  add_index "group_members", ["user_id"], name: "index_group_members_on_user_id", using: :btree
+
   create_table "groups", force: true do |t|
-    t.integer  "user_id",      null: false
-    t.integer  "target_id",    null: false
+    t.integer  "user_id",                     null: false
     t.string   "name"
     t.string   "display_name"
+    t.boolean  "private",      default: true
     t.datetime "created_at"
     t.datetime "updated_at"
   end
 
   add_index "groups", ["name"], name: "index_groups_on_name", using: :btree
-  add_index "groups", ["target_id"], name: "index_groups_on_target_id", using: :btree
   add_index "groups", ["user_id", "name"], name: "index_groups_on_user_id_and_name", unique: true, using: :btree
   add_index "groups", ["user_id"], name: "index_groups_on_user_id", using: :btree