you can do that by associating an item to each user on you metadata adapter (see MetadataProvider).

Please take a look to the Basic Instant Messenger Demo;
you can find the sources of the adapter in the Lightstreamer distribution under LS_HOME/DOCS-SDKs/sdk_adapter_java/examples/Messenger_Adapters/src_metadata/messenger_demo/adapters

NOTE: the metadata adapter of the demo is not complete; you should add user authentication implementing the notifyUser method and by adding the missing check in the checkIMName method.
NOTE: the demo assumes the client subscribes to the specific item. You may choose to use the same item name client-side and then translate it to the user-specific item on the getItems method.

take also a look to these discussions: http://www.lightstreamer.com/vb/showthread.php?t=61 & http://www.lightstreamer.com/vb/showthread.php?t=890