Is there no longer a recognised difference between “working with …” and “taking part in a conference/discussion with …”?
I guess it depends how extremist the groups are, and where the funding is coming from. For example, I’d attend a conference on animal welfare if PETA were taking part and attending, even if I wholeheartedly disagree with their methods and consider some of their aims extremist. If the conference was being run by them and would benefit them financially, then I might not attend, though if there was a preponderance of more reasonable groups attending and speaking, that I wanted to hear, then I might hold my nose and attend.
So this is a nuanced situation, in my opinion. To suggest that any groups that come together for discussion on a topic must therefore be considered to be “working together” is a false representation of what appears to be happening here. A reasonable group could just as easily listen to the points made, work out whether those points change their own viewpoints or strengthen them, and learn from the experience. That’s why discussion is important.