I've got a project hosted on GitHub which somebody has forked. On their fork, they've created a new branch "foo" and made some changes. How do I pull their "foo" into a new branch also named "foo" in my repo?

I understand they could submit a pull request to me, but I'd like to initiate this process myself.

Assume the following:

1. Because they forked my project, both our repos share the same 'history'
2. Although GitHub shows their project was forked from mine, my local repository doesn't have any references to this person's project. Do I need to add theirs as a remote?
3. I don't have a branch called "foo" yet - I don't know if I need to manually create this first.
4. I definitely want this pulled into a separate branch and not my master.

git remote add coworker git://path/to/coworkers/repo.git
git fetch coworker
git checkout --track coworker/foo

This will setup a local branch foo, tracking the remote branch coworker/foo. So when your co-worker has made some changes, you can easily pull them:

git checkout foo
git pull

Response to comments:

Cool :) And if I'd like to make my own changes to that branch, should I create a second local branch "bar" from "foo" and work there instead of directly on my "foo"?

You don't need to create a new branch, even though I recommend it. You might as well commit directly to foo and have your co-worker pull your branch. But that branch already exists and your branch foo need to be setup as an upstream branch to it:

git branch --set-upstream foo colin/foo

assuming colin is your repository (a remote to your co-workers repository) defined in similar way:

git remote add colin git://path/to/colins/repo.git