sample useage:
$ cd project
$ guix package --manifest=manifest.scm --profile=.guix-profile
$ guix shell # --profile=.guix-profile is implicit

do some stuff. realize that you want to bring in another package.

$ guix install --profile=.guix-profile additional-package

Then additional package is instantly available. This is especially useful to lisp programmers because, currently, bringing in an additional library involves restarting shell, lisp process, reloading source files, regenerating process state.