You can use
mount /dev/sda /mnt/usb -o gid=(GID of users group),mode=666
to mount it owned by the GID of the users group and/or use "mode=" to change the mode of the mount.
You probably can't mount it at boot though, because the proper modules need to be loaded before it maps the USB device to a pseudo SCSI device. You could add "noauto,users,rw" to fstab to stop it trying to mount at boot and alow users to mount and write.