Using Mercurial on Alioth
Creating a new public Hg repository (for a new project)
See Alioth/FAQ#vcs-repos, FusionForge will create a directory named /srv/hg.debian.org/hg/project-name owned by your project group which will be accessible by hg ssh or http for read-only access.
Within that directory, you can hg init/clone/whatever as many branches/repos as you like. Note that if this is not your main working copy, you can save Alioth disk space by doing a pull from your main working copy and then not doing an update. No working copy files are necessary for the server to function.
For instance, you'll be able to check out your branch though ssh with:
hg clone ssh://<user>@hg.debian.org//hg/<group>/repo
Note the double-slash above.
Anonymous users will enjoy read-only access with:
hg clone http://hg.debian.org/hg/<group>/repo
It is also possible to have repositories tied to your Alioth username rather than an Alioth project. If you are going to host a bunch of repos, perhaps one for each package you maintain, this could make sense.
Debian calls these "private" repos (private meaning per user; these are still openly published for everyone).
They will look like http://hg.debian.org/hg/private/<username>/repo and are stored at /hg/private/<user>/repo on the server.
You'll need to submit a support request to get one of these.