I believe you need to
chmod g+w rantest99.
chmod g+w dirname makes a directory writable by members of the group. This is what
chmod g+s dirname does:
info coreutils 'Directory Setuid and Setgid'
27.4 Directories and the Set-User-ID and Set-Group-ID Bits
On most systems, if a directory's set-group-ID bit is set, newly
created subfiles inherit the same group as the directory, and newly
created subdirectories inherit the set-group-ID bit of the parent
directory. On a few systems, a directory's set-user-ID bit has a
similar effect on the ownership of new subfiles and the set-user-ID
bits of new subdirectories. These mechanisms let users share files
more easily, by lessening the need to use 'chmod' or 'chown' to share