If you are using no AD then i guess server isnt governing user profiles and things, unless you are using nouvell or some crap like that.
You can get away with MSN messenger, although you will need to register each user with a .net passport.
This will take some time if your doing it for alot of systems, also people will be able to use this to communicate with external sources ... i.e. friends and family ... its a nightmare for security reasons too.