Yes, it could have transwomen and transmen but not normal men/women and not non binary people.
There would have to be a proportionate reason why you would exclude non-gender change people though. To exclude someone on the basis of a PC you would have to have a fair reason, like to discuss specific issues around gender change which they would not feel comfortable discussing with non gender change people. Or to increase the bird watching uptake of gender change people who are uncomfortable around non gender change people, feel judged etc.
i don’t think you can just exclude someone based on a PC without a good reason.
You can do a super inclusive bird watching walk for people who are super inclusive and want to celebrate diversity while looking at birds and it could be an interest group I guess, but you can’t exclude anyone on the basis of a PC like sex or gender change or age or whatever. Or you could do a private friends guided walk of whoever you like.
IANAL and this is just my understanding. Happy to be wrong.