diff --git a/app/controllers/members_controller.rb b/app/controllers/members_controller.rb
index 866ddca3a..540489b22 100644
--- a/app/controllers/members_controller.rb
+++ b/app/controllers/members_controller.rb
@@ -96,7 +96,7 @@ class MembersController < ApplicationController
   end
 
   def autocomplete_for_member
-    @principals = Principal.active.like(params[:q]).find(:all, :limit => 100) - @project.principals
+    @principals = Principal.active.not_member_of(@project).like(params[:q]).all(:limit => 100)
     render :layout => false
   end
 
diff --git a/app/models/principal.rb b/app/models/principal.rb
index f79b9df7a..2bf97d05c 100644
--- a/app/models/principal.rb
+++ b/app/models/principal.rb
@@ -32,6 +32,16 @@ class Principal < ActiveRecord::Base
      :order => 'type, login, lastname, firstname, mail'
     }
   }
+  # Principals that are not members of projects
+  named_scope :not_member_of, lambda {|projects|
+    projects = [projects] unless projects.is_a?(Array)
+    if projects.empty?
+      {:conditions => "1=0"}
+    else
+      ids = projects.map(&:id)
+      {:conditions => ["#{Principal.table_name}.id NOT IN (SELECT DISTINCT user_id FROM #{Member.table_name} WHERE project_id IN (?))", ids]}
+    end
+  }
 
   before_create :set_default_empty_values
 
diff --git a/app/views/projects/settings/_members.html.erb b/app/views/projects/settings/_members.html.erb
index e604566d0..531a7ccbc 100644
--- a/app/views/projects/settings/_members.html.erb
+++ b/app/views/projects/settings/_members.html.erb
@@ -50,7 +50,7 @@
 <% end %>
 </div>
 
-<% principals = Principal.active.find(:all, :limit => 100, :order => 'type, login, lastname ASC') - @project.principals %>
+<% principals = Principal.active.not_member_of(@project).all(:limit => 100, :order => 'type, login, lastname ASC') %>
 
 <div class="splitcontentright">
 <% if roles.any? && principals.any? %>
diff --git a/test/unit/principal_test.rb b/test/unit/principal_test.rb
index 5fd10d8ed..63f00bcd9 100644
--- a/test/unit/principal_test.rb
+++ b/test/unit/principal_test.rb
@@ -18,6 +18,13 @@
 require File.expand_path('../../test_helper', __FILE__)
 
 class PrincipalTest < ActiveSupport::TestCase
+  fixtures :users, :projects, :members, :member_roles
+
+  def test_not_member_of_scope_should_return_users_that_have_no_memberships
+    projects = Project.find_all_by_id(1, 2)
+    expected = (Principal.all - projects.map(&:memberships).flatten.map(&:principal)).sort
+    assert_equal expected, Principal.not_member_of(projects).sort
+  end
 
   context "#like" do
     setup do