Single Sign On

Hello there,
I have been looking for a solution to this one problem for at least 2 years and have yet to find a solution.

My issue is that I have several subdomains on one master domain.
domain.com
test.domain.com
test2.domain.com
ect…
I want to allow members to sign into domain.com and be able to use a feature on test.domain.com without having to login again. A example is IGN.com they use wordpress to host there blogs for each user using subdomains (like user.ign.com) without having to login again after login in to IGN.com.

Please help, I am somewhat familiar with wordpress and can add code if i know where and what to add. Also i do not what to use multi site. these sites (domain.com, test.domain.com ect…) are already set up and functional.

Thanks