Sometimes it is necessary to keep people from certain groups out of the conversation. They bring harmful ideas, are often abusive, and willfully ignorant to struggles faced by minority communities. It is important to point out that J K Rowling is not a feminist (Google ‘TERFs’ if you don’t know why). But gatekeeping is a slippery slope and one you should approach with great caution.
The most common forms of gatekeeping are towards those at the intersection of multiple minorities, or whose minority status is not obvious to all around them. Queer (and especially trans), Dalit and poor women being kept out of feminist circles; bisexual and asexual people being kept out of the LGBTQIA+ community (they are allegedly “straight people that pretend to be queer”, and yes, it is as ignorant as it sounds) and so on. The invaluable contribution of these communities to their respective movements are easily ignored.
Counterintuitively, the most common instance of wrongful gatekeeping doesn’t come from the right-wing or the more conservative sections of movements. It is usually done by people who are part of the oppressed community and passionate about the cause, but in the process attack others and undermine the broader movement. It typically comes from the section of the community that falls in the gap between those that are passionate about a cause and those that are willing to work for it. You must care about feminism to want to “protect” it from trans people, albeit not enough to do research and form a well-educated opinion.
Part of the problem is the romanticization of oppression by privileged members of the movement. The idea that movements created to fight systemic oppression are inner circles ignores the harsh reality of oppression that those that do not have the same experience can truly understand. For rich, upper-caste women, the idea of being feminist is often closer to something like a book club of like-minded people (this is still important!) than the front lines of a long, historical struggle for survival (you don’t turn people away from fighting with you for your cause unless you don’t think you need them).
Minorities are not the only victims of this gatekeeping, however. A large part of cancel culture stems from gatekeeping of allies to social justice movements. People that are willing to learn are rejected because of things they did in the past, or opinions that they were willing to change. Allies of movements are often critical for their success, and while it is unwise to bend a movement to find more allies, it is as dangerous to reject people that are willing to help altogether.
The key to being able to identify who to keep out of movements is to educate yourself. I recommend reading these to get a better idea of how wrongful gatekeeping works and how to fight it: