A 30-second search pulls up a dozen or so in my state alone that I would expect to not be allowed to attend as a cis-person, as I wouldn’t expect to be allowed into any kind of support group specifically for transpeople unless friends/family are invited. Those are groups designed and intended for transpeople, and my presence would likely make them feel at least uncomfortable, if not downright unsafe, which may prevent them from talking about the things they need to talk about, doing them a disservice.
It’s their safe space, and my being unwelcome there does not make them or the group “cisphobic,” nor would it upset me if I were turned away. In fact, I fully support and encourage such spaces.
Despite all the knee-jerk bullshit against safe spaces, everyone needs them for various reasons, and for those spaces to be effective, they need to set boundaries of who can and cannot attend. This is not a bad thing.