It won't matter if you're using Liferay in a cluster. His code will only give you the users logged into a given node, not the entire cluster. That list does not exist.

If you don't know the companyId thing, then you're probably too green as yet to be accessing the portal core to grab the list and display it within a portlet. All data stored in Liferay is keyed off of a company id (to support virtual hosting). You need to know what company id you're using so you can get the list of it's locally logged in users.

But honestly I can tell you that this is not a path that is going to give you what you want. There is no current list of logged in users because users do not have to log out (they can just close their browsers, their computer/browser can crash abruptly stopping any communication, network links can drop i.e. you take your phone out of cell range, ...).

Whatever list you think you're going to get and whatever you expect to do with it, well I can tell you that 99% of the time it will be incomplete and inaccurate.