In this link:
You can see an example of OrganizationAPI that give you access to information of GateIn.
It's an example about how to invoke there services from your portlet.
Here also you can see another example:
Of a portlet that uses these services.
Hope this helps.
I have one more doubt, in your examples you only explained about OrganizationService.
Is it possible to use other services in the same manner?