group.rb

class Group < ActiveRecord::Base
  has_many :users

  def email_regex
    #This can be generated here or pulled from a database
  end
end

routes.rb

ActionController::Routing::Routes.draw do |map|
  map.resources :groups, :has_many => :users
  map.resources :users, :except => [:new, :create] #This is so that the user can't be created unless going through a group
  
  #Other routes down here.
  
end

user.rb

class User < ActiveRecord::Base
  belongs_to :group
  
  validates_presence_of :group_id
  
  def validate
    errors.add(:email, "Your error message here") if !group || group.email_regex ~= self.email
  end
end

UsersController.rb

class UsersController < ApplicationController
  
  #only showing new, but you'll most likely have index, show, edit, destroy and all those
  
  def new
    @user = current_group.users.new
  end
  
  def create
    @user = current_group.users.new(params[:user])
    if @user.save
      flash[:notice] = "#{@user.full_name} created successfully"
      redirect_to(users_path)
    else
      flash[:error]  = "We couldn't set up that account, sorry.  Please try again, or contact an admin (link is above)."
      render :action => 'new'
    end
  end
  
  private 
  def current_group
    @current_group ||= Group.find(params[:group_id])
    #this will raise an error if no group is found, so you might want to catch that
  end
end