I had a look at the guide you linked to. What it's doing is having you put the public ssh key from each user who is going to be authorized to access the git repo into the /home/git/.ssh/authorized_keys file. What the guide isn't telling you is how to generate an ssh key pair (I guess it assumes you already know how or have already had each user do so).
Each user who is going to access the git repo needs to have run, at a minimum,
$ ssh-keygen -t rsa
That will generate an ssh key pair, a private key and a public key (id_rsa and id_rsa.pub) It's the id_rsa.pub file from each user that is then appended into the /home/git/.ssh/authorized_keys file. Wtih that, the users should then be able to ssh into the git user account without having to type a password.
Speaking of which, you haven't mentioned setting a password for the git user you created. To do that, as root:
enter a password for git (in the blind)
enter the password a second time when prompted (again, in the blind)
If you had switched to root and then created the /home/git/.ssh directory, then that directory would be owned by root, not user git. If that's the case, correct that with:
chown -R git:git /home/git
I see the guide sets up the actual git repo in /opt/git/