new_invitation.html.erb

<!-- /app/views/admin/users/new_invitiations.html.erb -->

<h2>Send invitation</h2>

<%= form_for @user, :url => send_invitation_admin_users_path do |f| %>
   
   <table style='width: 50%'>
     <tr>
   		<td><%= f.label :first_name %></td>
   		<td><%= f.text_field :first_name %></td>
   	</tr>
   	<tr>
   		<td><%= f.label :last_name %></td>
   		<td><%= f.text_field :last_name %></td>
   	</tr>
   	<tr>
   		<td><%= f.label :email %></td>
   		<td><%= f.text_field :email, :required => true %></td>
   	</tr>
   </table>
   
  <%= f.submit  "Send an Invitation" %>
<% end %>

routes.rb

# /config/routes.rb

devise_for :users, 
          :controllers => { 
          :sessions => "users_sessions", 
          :registrations => "user_registrations", 
          :passwords => "user_passwords",
          # Proper invitations should be sent through the active_admin interface.
          :invitations => 'users_invitations' # user_invitations_controller.rb
          }

users.rb

# /app/admin/users.rb

action_item do
  link_to 'Invite New User', new_invitation_admin_users_path
end

collection_action :new_invitation do
	@user = User.new
end 

collection_action :send_invitation, :method => :post do
	@user = User.invite!(params[:user], current_user)
	if @user.errors.empty?
		flash[:success] = "User has been successfully invited." 
		redirect_to admin_users_path
	else
		messages = @user.errors.full_messages.map { |msg| msg }.join
		flash[:error] = "Error: " + messages
		redirect_to new_invitation_admin_users_path
	end
end

users_invitational_controller.rb

# /app/controllers/users_invitations_controller.rb

class UsersInvitationsController < Devise::InvitationsController
   
   # GET /resource/invitation/accept?invitation_token=abcdef
  def edit
    render :edit, :layout => false
  end

  # PUT /resource/invitation
  def update
    self.resource = resource_class.accept_invitation!(resource_params)

    if resource.errors.empty?
      flash_message = resource.active_for_authentication? ? :updated : :updated_not_active                                                                                        
      set_flash_message :notice, flash_message
      sign_in(resource_name, resource)

      redirect_to '/profile', :alert => "Welcome! Please fill out your profile and upload a headshot." 
    else
      respond_with_navigational(resource){ render :edit, :layout => false }
    end
  end
end