How to pull remote branch from somebody else's repo

git remote add coworker git://path/to/coworkers/repo.gitgit fetch coworkergit 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 foogit 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

No, you don't need to add them as a remote. That would be clumbersome and a pain to do each time.

Grabbing their commits:

git fetch git@github.com:theirusername/reponame.git theirbranch:ournameforbranch

This creates a local branch named ournameforbranch which is exactly the same as what theirbranch was for them. For the question example, the last argument would be foo:foo.

Note :ournameforbranch part can be further left off if thinking up a name that doesn't conflict with one of your own branches is bothersome. In that case, a reference called FETCH_HEAD is available. You can git log FETCH_HEAD to see their commits then do things like cherry-picked to cherry pick their commits.

Pushing it back to them:

Oftentimes, you want to fix something of theirs and push it right back. That's possible too:

git fetch git@github.com:theirusername/reponame.git theirbranchgit checkout FETCH_HEAD# fix fix fixgit push git@github.com:theirusername/reponame.git HEAD:theirbranch

If working in detached state worries you, by all means create a branch using :ournameforbranch and replace FETCH_HEAD and HEAD above with ournameforbranch.

If antak's answer:

git fetch git@github.com:<THEIR USERNAME>/<REPO>.git <THEIR BRANCH>:<OUR NAME FOR BRANCH> 

gives you:

Permission denied (publickey).fatal: Could not read from remote repository.Please make sure you have the correct access rightsand the repository exists.

Then (following Przemek D's advice) use

git fetch https://github.com/<THEIR USERNAME>/<REPO>.git <THEIR BRANCH>:<OUR NAME FOR BRANCH>