I would create a new group, and add everyone to the group who should have dialout access. Select all users, choose 'Properties' from the User menu and disable dial-up on all accounts. Then do a 'Select Users' from the User menu and choose the new group you created. Get properties on all the newly selected accounts and give them dial-up access. Using this procedure you will be able to narrow down your dial-up access permissions without worrying about looking at every account.