I actually had this conversation with a friend of mine recently.
My personal opinion is that women's spaces are for women. It is a place for them to go and feel safe. Having a FTM or a male-identifying person there might make the ladies feel like they were no longer safe or that they had lost something. Personally I don't feel that going to women's group would harm my own identity, but I feel like it might harm theirs. In the end, I think it is a respect thing. I respect women enough to understand that they need their own spaces, just like men do. I was socialized as a woman (a southern belle one even) but because of how I identify, I feel that my presence would be harmful to them.
That being said, if you want to pull out of the group, I think you should be open with them. (If you feel safe, of course) If you are concerned about your own identity and you are questioning, I don't see the problem with being active in the org until you are sure, as long as you can be active and further the group's mission.
As for where do you go: a transfriendly group. GLBTQ groups are usually pretty accepting. I would join the one my Uni, but I am not going to until my husband quits his job (he is really active in campus life). Or you could join something that is not specfic to a certain group. Think honors groups for certain majors. I am in Phi Alpha Theta, the history club, and it is awesome.
Not everything in your life has to center around your gender identity. But, this is all my opinion and nothing else. Like Annah, I feel like if I jumped into a women's group I would be running away from myself a little.