Setting Up a Private Repository
If you need to setup a private repository and want to do so locally, rather than using a hosted solution, you have a number of options.
Repo Access over SSH
Generally, the easiest solution is to simply use Git over SSH. If users already have ssh accounts on a machine, you can put the git repository anywhere on the box that they have access to and let them access it over normal ssh logins. For example, say you have a repository you want to host. You can export it as a bare repo and then scp it onto your server like so:
$ git clone --bare /home/user/myrepo/.git /tmp/myrepo.git
$ scp -r /tmp/myrepo.git myserver.com:/opt/git/myrepo.git
Then someone else with an ssh account on myserver.com can clone via:
$ git clone myserver.com:/opt/git/myrepo.git
Which will simply prompt them for thier ssh password or use thier public key, however they have ssh authentication setup.
Multiple User Access using Gitosis
If you don't want to setup seperate accounts for every user, you can use a tool called Gitosis. In gitosis, there is an authorized_keys file that contains the public keys of everyone authorized to access the repository, and then everyone uses the 'git' user to do pushes and pulls.
