Central Login system on several CRE Loaded sites

We currently have 3 CRE stores. A site for Ireland, UK and South Africa. We will soon be launching several additional stores for other countries. Website: [login to view URL] At the moment a customer must register on the gift recipients home country site. If you need to send a gift to friends in Ireland, UK and South Africa you need to register 3 times. What we need is a central login system whereby a customer only need to register once on any of the country sites and use that login on any country site. Any ideas on how to achieve this? Perhaps there is a simply solution.