What version of GlassFish are you using?
Using version 2.1.1 (and many earlier releases) you can manage multiple instances within a domain from a single admin console. An instance is, basically, a Java virtual machine in its own operating system process running the server. With GlassFish v2.1.1 you can create multiple instances in a single domain and manage all of them using a single admin console.
Note that instances are quite different things from virtual servers. I'm sure the web team folks have a better explanation, but at a basic level you can think of virtual servers as http listeners running within a single GlassFish instance. As such, all virtual servers running in an instance share the same VM and are on the same host but can listen on different ports. That's why they are "virtual" servers, not real ones.
GlassFish v3.0 - just released last week - supports only single-instance domains so far. The plan is for a future release of v3 to support multiple instances as v2.1.1 does.
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