Extract the zip file.open the 3.x folder.Upload the groupquantitydiscount.ocmod.zip file from Extensions > Installer. Refresh the Modifications from Extensions > Modifications. Go To System > Users > User Groups to grant the permission. Then Go to Extensions.You will see the Group Quantity Discount. After successful install You can configure the settings as per your requirements. Then Go to Extensions > Total.You can Enable the Group Quantity Discount.

Extract the zip file.open the 2.3x/2.x folder.Upload admin folder to your server from the "Upload" folder.
Upload the install.ocmod.xml file from Extension Installer. Refresh the Modifications from Extensions > Modifications. Then Go to Extensions.You will see the Group Quantity Discount. After successful install You can configure the settings as per your requirements. Then Go to Extensions > Total.You can Enable the Group Quantity Discount.