Puppet can manage users (and a plethora of other things). I would warmly recommend any admin with more than 3 servers integrating puppet into their environment. The wiki on puppetlabs.com has many articles that will help you get up and running quickly. The advantages are definitely worth your time. Consider writing this piece of code once:
ensure => present,
group => ["sysadmin","dba"]
...and have it applied on many servers within a given timeframe.
And yeah, dont edit your shadow file by hand :)
There are also packages like cluster-ssh, but in my experience, while useful, the workflow with this type of software becomes uncomfortable with more than 3-4 servers - this is just screen real-estate wise. I am not even talking about various issues that will arise due to differences in server filesystem layouts (say bye-bye to tab completion), installed packages, etc