I am looking to change 'register' to 'join' in the wp admin bar for logged out users. How can this be achieved?
Are you referring to the meta bar where people can login or register? I'd recommend using jQuery, because if you make the change to the core, it'll be overwritten once you update it.
This basically states find an anchor tag where the href contains the word register. Once it finds it, it changes the inner text to 'join'.