Group Membership Discounts using Group Based pricing
You can use webform_civicrm module - but it will be quite some config! 

Something like:


add a Drupal field - how many members would you like to sign up?
make as as many contacts in the civicrm tab as you need (you may need to bump up a parameter in webform_civicrm code) - you can make one (add all the fields you want) then in the webform tab -> clone
use conditionals to show the correct number of contacts 
make sure you have relationship to primary contact (something like Member sign-up by)
make sure your membership type filters down to inherit by relationship
the monies... will be the default $ in your Membership Type config - however - you can add your own contribution field in webform and use conditionals (or jQuery to get the grand total $ right - based on the # of members field the user selected


Good luck setting that up! Please do report back!