NPM has npm install <folder>. JSPM requires more work.

Link a project...

cd <project-root>
jspm link github:some/repo@branch -y

...then install from that link

cd <other-project-root>
jspm install --link github:some/repo@branch -y

Some notes:

project-root is the directory that contains the project.json file.

-y forces an overwrite of the local github registry with the newest version of the package. Do this each time the package changes.

See also:

https://github.com/jspm/registry/wiki/Configuring-Packages-for-jspm

http://jspm.io/docs/linking.html