Activate developer mode.

Extract the id's of the users you want to add to the sale group by going to the Users list (Settings > Users) and exporting the data for the users.

Go to the Security Groups list (Settings > Groups) and export the record for the sale group (the Id and Users fields only) to a import compatible csv file.

Update the users column in the csv file with the id's of the users separated by commas.

Import the csv file in the Security Groups list.