About the form_for option

<%=form_for(resource, as:resource_name, url:registration_path(resource_name))do|f|%>

<div class='register_back'>
  <div class='log-up'>
    <%=render partial: 'common/login_logo'%>

    <div class='log-in-inner'>
      <h2>Membership </h2>

      <%=form_for(resource, as:resource_name, url:registration_path(resource_name))do|f|%>
        <%=device_error_messages!%>

        <div class='form-group'>
          <%=f.label: email address %><br/>
          <%=f.email_field:email,autofocus:true,class:'form-control'%>
        </div>

        <div class='form-group'>
          <%=f.label: Password %>
          <%if@validatable%>
            <em>(<%=@minimum_password_length%> characters or more)</em>
          <%end%><br/>
          <%=f.password_field:password,autocomplete:'off',class:'form-control'%>
        </div>

        <div class='form-group'>
          <%=f.label: Password (confirmation) %><br/>
          <%=f.password_field:password_confirmation,autocomplete:'off',class:'form-control'%>
        </div>

        <div class='form-group'>
          <%=f.label: Group name (half-width alphanumeric) %><br/>
          <%=f.text_field:group_key,class:'form-control'%>
        </div>

        <div class='actions'>
          <%=f.submit class: 'btn btn-primary with ripple', value: 'New registration'%>
        </div>
      <%end%>

      <%=render'device/shared/links'%>
    </div>
  </div>
</div>