Many standards packages for sun are available at http://www.sunfreeware.com/
Navigate to solaris10 x86 or sparc depending on your architecture,
and search for sudo in the list presented.
you may find it will need one or two other packages from there as well. These packages are installed using the pkg_add tool in solaris. Once sudo is installed (in /usr/local) you can configure as you would a normal sudo, config file is in /usr/local/etc/sudoers if I remember correctly.
however, if you're willing to entertain a method on both platforms that doesn't involve elevated (root) privilege, what you're wanting to do can more simply be achieved by setting /data to a particular group that both accounts are a member of, and setting group write flag using chmod g+w /data. Group semantics should honor that flag for new files and directories created underneath. To chgrp exsting files you would do :
chgrp -R common_group /data
chmod -R g+w /data