How do I auto mount a USB drive that all users can write to?

You can do a chmod after you mounted the partition, but that wouldn't be persistent accross reboots.

You should try this fstab line:

/dev/sda1   /media/josiah    vfat    user,umask=0000   0   0

Or this mount options:

mount -t vfat -ouser,umask=0000 /dev/sda1 /media/josiah

That will make the mounted partition world readable and writable.

If you need a less permissive setup, you should create a new group and mount as follows:

mount -t vfat -ouser,gid=1010,umask=0007 /dev/sda1 /media/josiah

It assumes your new group's gid is 1010. All users that need access to the mountpoint will need to be added to the new group.

Edit the permissions for the mount directory.

In your case, chmod 777 /media/josiah ought to do the trick quite nicely.