Who are these members?

All the search engines (google, bing etc) run robots across the whole internet. It's how they become aware of the things they report as search results.

Forums like Priuschat have interfaces specifically for those robots, to make their content searchable and make the results more useful. Those interfaces also manage the robots- keeps them from taking too many resources away from the rest of the site.

Result: the user index can tell the difference between guests, logged in users, and search engine robots that are using their own dedicated interface to discover content.

The robots you might be concerned by are the ones that mimic human user behavior. They would just show up as "guests" in that panel, because it can't tell the difference between them and real humans.

 

There's actually more robots using the internet than real people. One time I had a website called iRobot.com to track their advancements and I left comments open without registration and watched them spam the website to death. Everytime a robot posted a spam comment it alerted all the other robots via email who then came and posted their own spam. Once I hit about 100K spam robot comments, my webhost contacted me and said my site was sending out more than a 1000 emails an hour and I had to shut it down.

The one most to blame for this ruining of the Web is Google trying to sell ads. They've entirely corrupted search engine rank for the sole purpose of advertising dollars and it gets worse and worse as time goes by. It used to be you could ask Google how to learn to do a task/develop a skill and you'd get websites that'd teach you. These days they just try to sell you a product or service and the DIY links get buried.