It isn't about appearance, that is a superficial issue.
It is possible, or maybe even likely that a woman who has chosen to transition will have experience male violence as a woman. And so the shared experience of male violence is the basis of the group.
Also, m
More often than not, when women talk about why they transitioned, if not acts of violence by men, but being objectified, means that for many getting rid of their breasts which men have leered atand objectivifed them is a relief.
And not saying I know, but just as many women say that even if a man who has transitioned "sucessfully" ie achieving an a constrcted representation of a woman women can still see their maleness, so as likely that a woman who has transitioned and "appears" male there is likely to be a sense of them as a woman.
And I suspect those organising the event will have a process for ensuring those attending are not creating a negative environment.
ie if a woman who has transitioned chose to swaggered around in an exaggerated male way, or something, they would be asked to modify their behaviour or leave.