Per the login page:

“Not a user yet? Signup is by invitation only to combat spam and increase accountability. If you know a current user of the site [1], ask them for an invitation or request one in chat [2].”

[1] https://lobste.rs/u/

[2] https://lobste.rs/chat

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Related comment on the history of lobste.rs:

https://news.ycombinator.com/item?id=23441470