I\'m an SVN user at the moment and looking at moving my projects to git.
I use one SVN repository to hold all the projects I create. My structure is something like this:
In Git, it is better to have each project in its own repo, and the libraries in another repo if needed and used as submodules ( equivalent of svn externals) in each project.
This is because in Git, a repo is a much more lightweight concept than SVN and also, more importantly, there is no ability to clone individual folders ( not to be confused with sparse checkout) within a repo separately like you are able to checkout and work on individual folders in SVN. So if you had all projects in a single repo, you will have to clone them all.
Serving Git repos, using smart-http, git daemon and ssh is pretty straightforward. There is also Gitolite for managing multiple repos ( including authorization and authentication). Read the linked chapter on ProGit on serving Git repos- http://progit.org/book/ch4-2.html
Coming to your grouping, you can put the repos in folders as per your grouping structure, and serve using, say smart http method, whereby the repo urls will look like the urls that you would have used with SVN, will all projects seeming to be under the grouping etc.