Create a single registration/ sign on (SSO) for two website -- 2

One software is a dating service while the other a free email service. both websites resides on the same server and share a domain name. This task requires you to make both software utilise 1 single sign on. Users will be able to utilise 1 single registration which is replicated to the other. Sign on will be authenticated to the other so upon login user can click a link to redirect/load the other app without having to register or re-login again. When you apply to this job, please tell me how you will achieve this aim and also provide where available, sample of a relevant job you have completed. NOTE: I will never award this project to a beginner and I will never award it to anyone who does not have adequate experience.