Yes, you can add a user easily with the built in GUI tool that ships with OS X, but that will setup the account to be able to share music, login, and even modify the dock. If you want to setup a captive account, you will need to be able to setup an account from scratch.
However, most blog postings are so old and outdated, they are simply wrong. This is the updated procedure, wherein I setup a captive git account:
sudo dscl . -create /Users/git UserShell /usr/bin/git-shell
sudo dscl . –passwd /Users/git PASSWORD
sudo dscl . -create /Users/git UniqueID 503
sudo dscl . -create /Users/git PrimaryGroupID 1000
sudo dscl . -create /Users/git NFSHomeDirectory /Users/git
sudo mkdir git
sudo chown git git
That should be enough to get you started.