Not all rationalists are EAs, and not all EAs are rationalists, but there are many people who are both. The very idea of using evidence and numbers to choose the optimal charity has a rationalist vibe. But of course, one can donate to an effective charity without being interested in LessWrong-style rationality.

https://www.lesswrong.com/ and https://forum.effectivealtruism.org/ run on the same software, and some people have an account at both of them, but they are generally separate web communities.