Add login Function to Joomla 2.5 site

I need to add a function to a Joomla 2.5 website. I have a site where I host my draft sites while I am developing them for clients. [login to view URL] As you can see there are a number of places inviting clients to "log in" Once the user has logged I want the site to automatically redirect them to the subdomain where I have set up their draft site. All the while the URL remaining with [login to view URL] Excellent communication skills are required. Thanks