project.rb

class Project < ActiveRecord::Base
  attr_accessible :description, :name
  
  has_many :user_projects
  has_many :users, :through => :user_projects
end

user.rb

class User < ActiveRecord::Base
  attr_accessible :email, :password, :password_confirmation
  has_secure_password
  validates_uniqueness_of :email
  
  has_many :user_projects
  has_many :projects, :through => :user_projects
end

userproject.rb

class UserProject < ActiveRecord::Base
  belongs_to :project
  belongs_to :user
end