EA as a movement was a combination of a few different groups (This account says Giving what we can/80000 hours, Givewell, and yudkowsky's MIRI). However, the main source of early influx of people was the rationalist movement, as Yud had heavily promoted EA-style ideas in the sequences.

So if you look at surveys, right now a a relatively small percentage (like 15%) of EA's first heard about it through lesswrong or SSC. But back in 2014, and earlier, Lesswrong was the number one onroad into the movement (like 30%) . (I'm sure a bunch of the other answers may have heard about it from rationalist friends as well). I think it would have been even more if you go back earlier.

Nowadays, most of the recruiting is independent from the rationalists, so you have a bunch of people coming in and being like, what's with all the weird shit? However they still adopt a ton of rationalist ideas and language, and the EA forum is run by the same people as Lesswrong. It leads to some tension: someone wrote a post saying that "yudkowsky is frequently confidently, egregiousl wrong", and it was somewhat upvoted on EA forum but massively downvoted on Lesswrong.