group.rb

class Group
  module Error
    class Standard < StandardError; end
    class AlreadyAMember < Standard; end
    class NotPermittedToJoin < Standard; end
  end

  def join user
    raise Error::NotPermittedToJoin unless self.permitted?(user)
    raise Error::AlreadyAMember if self.member?(user)
    self.members.create :user => user
  end
end

group_error_display.rb

module GroupErrorDisplay
  @@errors = {
    Group::Error::AlreadyAMember => "You are already a member of this group.",
    Group::Error::NotPermittedToJoin => "You don't have the proper permissions to join this group."
  }

  def self.message(error)
    @@errors[error.class]
  end
end

groups_controller.rb

class GroupsController < ApplicationController
  def join
    @group = Group.find(params[:id])
    @group.join current_user
    flash[:notice] = "Welcome to the Group!"
    redirect_to @group
  rescue Group::Error::Standard => exception
    flash[:error] = GroupErrorDisplay.message(exception)
    render :action => 'request'
  end
end