calling the mailer

foo.rb

@user = User.invite_without_notification!

@group = Group.find_by_name('foo')

@group.memebers << @user

GroupMailer.welcome_invitation(@group, @user).deliver

group_mailer.rb

class GroupMailer < ActionMailer::Base
  default :from => "support@example.com"

  def welcome_invitation(group, user)
    @group = group
    @user = user
    mail(:to => user.email, :subject => 'A group has been shared with you')
  end
end

welcome_invitation.html.erb

<p>Hello <%= @user.email %>!</p>

<p>Someone has invited you to join a group called <%= @group.name %> on <%= root_url %>, you can accept it through the link below.</p>

<p><%= link_to 'Accept invitation', accept_invitation_url(@user, :invitation_token => @user.invitation_token) %></p>

<p>If you don't want to accept the invitation, please ignore this email.<br />
Your account won't be created until you access the link above and set your password.</p>