app/views/devise/invitations/edit.html.erb
<h2>Set your password</h2>
<%= form_for resource, :as => resource_name, :url => invitation_path(resource_name), :html => {:method => :put, :class => "form-horizontal"} do |f| %>
<fieldset>
<%= devise_error_messages! %>
<%= f.hidden_field :invitation_token %>
<div class="control-group">
<%= f.label :password, :class => 'control-label' %>
<div class="controls">
<%= f.password_field :password, :class => 'text_field' %>
</div>
</div>
<div class="control-group">
<%= f.label :password_confirmation, "Confirm new password", :class => 'control-label' %>
<div class="controls">
<%= f.password_field :password_confirmation, :class => 'text_field' %>
</div>
</div>
<div class="form-actions">
<%= f.submit "Set my password", :class => "btn btn-primary" %>
</div>
</fieldset>
<% end %>