One option to address this scenario:
DE-activate your project members, and THEN assign them the tasks. It seems counter-intuitive, but you may assign tasks to inactive project members. (The DE-activation may be accomplished through the Members -- List View menu; check the boxes, then select the Checked Item Actions Menu...).
Once you have satisfied yourself that your schedule is complete and you are ready to notify the assignees, simply RE-activate all the members, and they will receive a single "Welcome Back to the Project" sort of notification.