amalloy's suggestion didn't work for me. This did:
git remote add theirusername https://github.com/theirusername/reponame
git fetch theirusername
git checkout -b mynamefortheirbranch theirusername/theirbranch
Resources:
- https://git-scm.com/book/ch2-5.html
- http://crunchify.com/how-to-fork-github-repository-create-pull-request-and-merge/