I’m not sure about this, and it would be far better to contact someone at the top for advice. However, I would interpret “We ask that all work and activities of the grove should be kept confidential to that grove and its membership and not be discussed in the wider public unless by mutual agreement of its members,” as being connected to (perhaps) data protection and privacy laws, which are quite strict in Europe, including the UK. I don’t think confidentiality means that you cannot publicise anything that you do, but you need consent and you need to respect people’s (legal) right to remain private. For example, if you posted a photograph of your group in which people were identifiable, you would need to have permission from those people to do so and so on.
I might be totally wrong on this one, so don’t take my word for it, but that is indeed what my understanding would be.