I mean they purchased a building for their HQ and had group meetups and stuff. It's a bit less nebulous than your spiderman fans example.
Similarly, effectivealtruism.org is just one non-profit in an ecosystem of dozens and does not own the trademark "effective altruism" or "effective altruist" (which might give it some ability to control how the trademark is used or by whom it is used).
You're overgeneralizing too far. This is like saying just because a church has a building doesn't mean they are an organization. The building was purchased with the expressed intent of being used for the group. It appears you trying to suggest there are weak ties where there are actually strong ones. These people all know each other and shared ideas.
I'm not a huge fan of EA, but I'm sick of seeing HN and EA endorse tribalism by saying that all Christians this or all Republicans this or all white people that or all rich people that....
-rich white Republican Christian