It's a lot of things. I'd say that at its heart it's a way of life, or a way to live life. That way manifests itself in many ways such that we can talk about common Buddhist values, world models, practices, community forms, etc. but all of those are implementation details of how you bring about something deeper, more subtle, and more fundamental than any of them. It's a little hard to point at what that way is, though, so that I can say some words about it, because whatever words I say it will not be the thing itself, like the way a finger pointing at the moon is not itself the moon. If I had to pick some very few words to capture the essential nature of the Buddha way, I would say that it asks us to be here, now, in our totality, fully engaged in the act of living as compassionate agents embedded in the world.

Hmm, I'm not sure. The group is set to be publicly visible so anyone should be able to find it and ask to join, although it's a "private" group meaning only members can see who else are members and can see posts. The link is live and works for me, so I'm not sure. As an alternative you can search "Buddhists in Effective Altruism" on Facebook and that should find the group.