Use this menu to assign more affiliates to more offers with one click. On this page you will have two lists of Affiliates and two lists of Offers. For each of them choose desired elements from Available list and move them to the Selected list. Once done click on the Mass assign button and all selected Affiliates will be assigned to the selected Offers.
Keep in mind that when you assign new affiliates to some offers, the already assigned affiliates won't be de-assigned. For example an offer has an affiliate. If you will assign two new affiliates using mass assign, the offer will now have 3 affiliates, one previously assigned and these 2 new affiliates.