hi
you normally don't do this.
you just do a rewrite in your apache or nginx - alternativly you can try setting up wildcard reverse-proxies (but i don't know if this will work :D)
so that allen.domain.com will be internally be handeled as domain.com/users/allen
so only one database-server is needed with new Databases for each user. which should be created via script. (and if you have a script, you can even create new config-files for your service, but i wouldn't recommend this)