Borrowing from answers in this Stack Overflow question, here's how you can get through a corporate (Microsoft) NTLM Proxy to clone git repositories from drupal.org:
cd into your drupal contrib directory (or wherever you want to put the repository).
$ export http_proxy="http://username:password@proxy:port/"
$ git clone http://git.drupal.org/project/[projectname]
Basically, you're first setting an environment variable to tell your shell to use an HTTP proxy, with your username/password combo. This variable will be used when making connections to git.drupal.org (and other services, like github). You can also set this in your
.bash_profile so it will be saved for future Terminal sessions.