No, I think you have misunderstood.
You can't offer a service for women (as defined by sex) and anyone who identifies as a woman, because then you are discriminating against men, who unlike women, don't have the option of membership if they don't identify as women.
'Idenitifying as a woman' is not a protected characteristic, so you could have an organisation for anyone who 'idenitifies as a woman', regardless of sex, as long as that were defined sufficiently widely to not indirectly discriminate against men.
However, at that point you might as well just have an institute for people who like crafting and meetings with invitied speakers.