member.rb

class Member < ActiveRecord::Base
  belongs_to :project 
  belongs_to :users
  attr_accessible :project_id, :user_id
end

new.html.erb

<%= simple_form_for(@project) do |f| %>
  <%= f.input :projectname %>
  <%= f.label "Select Client" %>
  <%= f.collection_select(:user_id, User.find(:all, :conditions => ["flag = 3"]), :id, :name, :class =>"user") %>
  <%= f.hidden_field :created_by, :value => current_user.name %>
  <%= label_tag(:description, "Description :") %>
  <%= f.text_area(:description, :cols => 5, :rows => 5) %>
  <%= f.input :start_date, :label => "Start date", :required => true %>
  <%= f.input :end_date, :label => "End date", :required => true %>
  <h5>Members</h5>
    <%= f.simple_fields_for :members do |s| %>
      <%= s.check_box :user_id, {}, u.id %><%= u.name %>
    <% end %>
  <%= f.submit "save", :class => 'btn btn-huge btn-info btn-embossed mlm' %>
<% end %>

project.rb

class Project < ActiveRecord::Base
has_many :members
belongs_to :user
attr_accessible :projectname, :user_id, :clientname, :created_by, :description, :start_date, :end_date, :members_attributes
accepts_nested_attributes_for :members, :allow_destroy => true 
end

projects_controller.rb

class ProjectsController < ApplicationController  
  def index 
    @projects = Project.all 
  end
  def show
    @project = Project.find(params[:id]) 
  end
  def new
    @use = User.where('flag = 1 or flag = 2')
    @project = Project.new
    @project.members.build
  end
  def create
    @project = Project.new(params[:project]) 
    if @project.save
    redirect_to @project, notice: 'Project Created Successfully.'
    else
      render :action => 'new'
    end
  end
  def edit
    @use = User.where('flag = 1 or flag = 2')
    @project = Project.find(params[:id])
  end
  def update
    @project = Project.find(params[:id])
    if @project.update_attributes(params[:project])
      redirect_to @project, notice: 'Project Updated Successfully.'
    else
      render :action => 'edit'
    end
  end 
  def destroy
    @project = Project.find(params[:id])
    @project.destroy
    redirect_to projects_url, notice: 'Project Destroyed Successfully.'
  end
end