Hi,
You have to tell 'mount' which (non-root) userid and groupid to use as options to the mount command :
mount /dev/hdc5 -t nfts -o uid=user1,gid=group1,ro /mnt/wherever
You can also use the umask option if you wish :
-o uid=user1,gid=group1,umask=027,ro
Hope this helps