Joomla PHP and MySQL Project

I have a joomla site. 1 main site with 5 subdomains all using same database with different table prefix. I want the login on any other site or subdomain if called, to be redirected to a named external subdomain and once login, to return user to called url. Similar to yahoo login e.g The main site is [login to view URL] and the subdomains are [login to view URL] , [login to view URL], [login to view URL] ...... and [login to view URL] Anytime a user profile or login is called, user will be redirected to [login to view URL] and once successfully login, user will be returned to ''called url'' The site is on shared server and clean code is expected. Also, I need user to be redirected to named url based on country IP Should you need further details, pls get in touch