In England and Wales, a charity is an organisation that is:
established for charitable purposes only, and subject to the High Court’s charity law jurisdiction
Purposes are what your charity is set up to achieve - they are explained in your governing document. To be charitable, your charity’s purposes must:
fall within the descriptions of purposes
be for the public benefit
As to what the 'public benefit' means, you can read up here
The legal opinion on what public benefit means is:
“To ascertain whether a gift constitutes a valid charitable trust so as to escape being void on the ground of perpetuity, a first inquiry must be whether it is public - whether it is for the benefit of the community or of an appreciably important class of the community. The inhabitants of a parish or town, or any particular class of such inhabitants, may, for instance, be the objects of such a gift, but private individuals, or a fluctuating body of private individuals, cannot.”
I suspect that our members would be 'a fluctuating body of private individuals'.