Integration with Java Portal

I am currently running G2 and Liferay Portal (a Java based portal). These are completely separate apps. At a bare minimun, I would like to be able to pass a username/password as parameters from a page in my portal to G2 to automatically log users in. In this type of approach, I would like to perform a post so the password is not visible in the URL. Ideally I would like to be able to somehow sync the sessions between the two apps so when a user logs out of the portal, they are also logged out of G2.

Please keep in mind that I am quite comfortable working in Java, but I have just started looking at PHP.

Has anyone done anything like this? Any suggestions would be greatly appreciated.