Lol I knew you would be talking about transgender people!
I believe this is more complex as you obviously have two groups (male and female).
I don't think they have a mental illness. I also believe that there are some transgender groups around too.
There is no discussion that their history will be very different from the people in the female group. Different is not to say that it has been easier or that they haven't been discriminated. Just that the reason they faced discrimination are different (until they had a sex change at least).
Whether you can 'incorporate' transgender into a female group depends on the outlook of transgender people. Do they see themselves as transgender only or do they see themselves as women?
For me, a transgender who has the operation and is now living as a woman will face the same discrimination now than a woman. Their history will be different but I have no doubt that they will also have incorporated to who they are, beliefs on what it means to be a woman.
A transgender who has not had the operation ...., it will depend a lot on whether they identify themselves as a woman or as androgynous, hermaphroditic, intersex, transsexual, third gender, bigender etc... I would only consider that their presence is OK if they define themselves as a woman (but then I don't see the others coming to a conference on women's right conference. I might be wrong though).
I am a bit baffled as to why a women's right conference should be only for women though just as I am baffled by the BLC being only for black people.