Demograph is a little free app that estimates the gender demographics of events on Meetup.com. It's primarily conceived and written by this guy, but credit where credit is due: Demograph is possible using the Meetup API and Gendered, a Ruby gem that uses genderize.io.
Demograph uses the first name of RSVP'd attendees ONLY to predict gender. It is not based on user-entered data on Meetup (which, while the ultimate source of truth, is not public), photo recognition, or any other methods. The goal is to be roughly accurate in the aggregate (i.e. at the event-level) but not necessarily at the individual level. Please don't be offended if your gender is miscategorized!
Additionally, Demograph makes a major simplifying assumption about gender: that it is binary. This is primarily due to technical challenges - I have not seen any statistics or APIs that would allow predictions of gender-queer attendance. But I definitely do not want to erase or ignore those for whom male or female labels don't fit. If you have ideas for how to make Demograph more inclusive, please let me know on the feedback page.