I assume you are limited to one available port on a static address that you want to use for multiple Rails applications.
You can accomplish this by configuring Apache so that it talks to the correct mongrel instance based on the incoming request.
Example: .xyz Domain Names | Join Generation XYZ would go to mongrel instance on port xxx and .xyz Domain Names | Join Generation XYZ would go to mongrel instance on port yyy.
A simpler way to do this is to use mod_rails. Configuring multiple Rails app in subdomains is quite easy with mod_rails. http://www.modrails.com