They need your IP at least, age and stuff.
It's not surprising that OPs friends can't join because they are out of the FBI's zone.
As for the source, the owner of this site also owned or still owns (not entirely sure) the biggest incel forum, and was investigated and interviewed by the FBI. In order to keep control he had to give over some permissions and/or information. That is what is known 100%. There is a bunch of speculation regarding FBI's involvement outside of that such as fake accounts on the forum and discord, etc.
It's all well known knowledge in that realm of the internet, but they delete or lock threads that mention it here.