It's closely tied into the DW code, not a standalone, but the code is here:

https://github.com/dreamwidth/dw-nonfree/blob/develop/cgi-bin/DW/Controller/Interface/Github.pm

Note: for it to work, you'll need to have that person be part of a team on your Github repo. What this adds is the ability for someone who is a contributor *but doesn't have commit access* to assign themselves.