Basic terms and concepts
After you make a change to the bsp, you need to either uprev or re-emerge it for build_packages/build_image to pick up the changes. (We realize this isn't ideal: See http://crbug.com/391962)
Use cros deploy
First, check if the third party library is already available as a Gentoo package under src/third_party/portage-stable or src/third_party/chromiumos-overlay.
If yes, then just add the package name to the DEPEND (if build-time dependency) and/or RDEPEND (if run-time dependency) of your app's ebuild.
Add it as an RDEPEND to chromeos-base/chromeos-bsp/chromeos-bsp-.ebuild in your overlay.
Example BSP that pulls in peerd and bash on the target.
CrOS uses the upstart system to manage boot and service startup. Simply add an upstart job for your service and install it into /etc/init
Example upstart script: link
system-services, unless your job in the critical boot path (e.g. network driver setup), in which case boot-services is appropriate.
The following additional resources go into more detail: