Tunaman suggested a Who’s online functionality. There is a supported Discourse plugin for this but it comes with some performance warnings:
Should I proceed and install this knowing we may need to uninstall later for performance issues?
Always hate to get used to a feature that gets yanked later on but we won’t really know the impact till we try right?
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OK so 2 votes we try 
One more and I’ll look to add it tonight after 10PM (there will be about 15 minutes of downtime to relaunch)