The docs show this for an example of a query for a project:
And after examining the returned data I can see that it returns info for this particular project:
Can I go the other way around and provide it with the normal URL instead of "project-6"? How do I know what project number a particular project is?
"project-6" is the project ID.
If you view the XML variant you will see an easy way to build the project URL:
Since they have started addressing projects by project ID instead of name we might have an opening on what we have complained a lot about: We cannot rename projects or have two projects with identical names.
I don't think that quite answers the question, Bjorn. For example, what's the project ID for http://www.geni.com/projects/Earls-of-Arundel ?