Just like HN grew around the writing of Paul Graham, the "rationalist community" grew around the writings of Eliezer Yudkowsky. Similar to how Paul Graham no longer participates on HN, Eliezer rarely participates on http://lesswrong.com anymore, and the benevolent dictator for life of lesswrong.com is someone other than Eliezer.

Eliezer's career has always been centered around AI. At first Eliezer was wholly optimistic about AI progress. In fact, in the 1990s, I would say that Eliezer was the loudest voice advocating for the development of AI technology that would greatly exceed human cognitive capabilities. "Intentionally causing a technological singularity," was the way he phrased it in the 1990s IIRC. (Later "singularity" would be replaced by "intelligence explosion".)

From 2001 to 2004 he started to believe that AI has a strong tendency to become very dangerous once it starts exceeding the human level of cognitive capabilities. Still, he hoped that before AI starts exceeding human capabilities, he and his organization could develop a methodology to keep it safe. As part of that effort, he coined the term "alignment". The meaning of the term has broadened drastically: when Eliezer coined it, he meant the creation of an AI that stays aligned with human values and human preferences even as its capabilities greatly exceed human capabilities. In contrast, these days, when you see the phrase "aligned AI", it is usually being applied to an AI system that is not a threat to people only because it's not cognitively capable enough to dis-empower human civilization.

By the end of 2015, Eliezer had lost most of the hope he initially had for the alignment project in part because of conversations he had with Elon Musk and Sam Altman at an AGI conference in Puerto Rico followed by Elon and Sam's actions later that year, which actions included the founding of OpenAI. Eliezer still considers the alignment problem solvable in principle if a sufficiently-smart and sufficiently-careful team attacks it, but considers it extremely unlikely any team will manage a solution before the AI labs cause human extinction.

In April 2022 he went public with his despair and announced that his organization (MIRI) will cease technical work on the alignment project and will focus on lobbying the governments of the world to ban AI (or at least the deep-learning paradigm, which he considers too hard to align) before it is too late.

The rationalist movement began in November 2006 when Eliezer began posting daily about human rationality on overcomingbias.com. (The community moved to lesswrong.com in 2009, at which time overcomingbias.com became the personal blog of Robin Hanson.)
The rationalist movement was always seen by Eliezer as secondary to the AI-alignment enterprise. Specifically, Eliezer hoped that by explaining to people how to become more rational, he could
increase the number of people who are capable of realizing that AI research was a potent threat to human survival.

To help advance this secondary project, the Center for Applied Rationality (CFAR)
was founded as a non-profit in 2012. Eliezer is neither an employee nor a member of the board
of this CFAR. He is employed by and on the board of the non-profit Machine Intelligence
Research Institute (MIRI) which was founded in 2000 as the Singularity Institute for Artificial Intelligence.

I stress that believing that AI research is dangerous has never been a requirement for posting on lesswrong.com or for participating in workshops run by CFAR.

Effective altruism (EA) has separate roots, but the two communities have become close over the years, and EA organizations have donated millions to MIRI.