OK, so you have an abstract (NON-POSIX) group list out there somewhere, and you want to use that list under puppet to customize the sudoers file on a range of systems?
So first up, don't. Stick with user groups, I expect there's some way to keep this simpler model.
But after that, if this really is what you're going to do (and TBH I can see why it's not so bad - you have to make some sort of arbitrary mapping at some point, so maybe this point is OK) what is the association between the system and the user? It seems like an extremely puppet based solution is pretty simple. First thought to use an external arbitrary text list is to use an extlookup - http://docs.puppetlabs.com/reference...html#extlookup
within a simple template file, but plenty of other solutions are also out there. I like using puppet-dashboard for this finer detail of config, in which you could add all the nodes within dashboard and use external node processing in puppet to pull in these little scrappy bits of data which you can manage fairly painlessly through a web interface.