Gatekeeping I feel, is a very case-by-case basis. I feel any group should be allowed to use discretion at who may be admitted, though I also agree that a lot of the time, people use their discretion far more stringently than what may be required, and they come off as control freaks or elitists.
Being as aspiring biologist, I'll use an analogy relevant to my field. Cells have to regulate what gets through the cell membrane. Various protein complexes in that membrane dictate what gets in and out depending on a variety of things, such the needs of the cell (basic things like water and nutrients), the cell's specific purpose (for example, a neuron (nerve cell) will have different needs than a myocyte (muscle cell), a specific point in the cell's life cycle, etc.
A cell needs to bring in new material to keep itself alive. Many types of dissolved nutrients need to come in through active transport. The process requires energy, as ATP hydrolysis is required for a membrane protein to change conformation (or shape), opening up to the outside environment and taking in a solute particle (a nutrient), then changing conformation again to let the particle in question into the cytosol – the inside of the cell. It's specialized of course – the proteins have a receptor site that recognizes certain particles and binds to them when the protein is open to the outside environment, then releases the particle into the cytosol when its conformation changes back. Without this process, the cell may not be able to get all the nutrients it needs. Glucose and sucrose, essential to the cycle of cell metabolism, cannot easily permeate the cell membrane by themselves. Essential ions, like potassium+, cannot get in. The cell will die.
Likewise, a lot of fandoms and groups I've been in have withered and died because the barrier of entry was a little much. Some people are content with this, taking the maxim "you either die a hero or live long enough to see yourself become the villain" a little too close to heart. But all too often, someone who has genuine interest and means no harm is turned away because of paranoia, and frankly, because it takes energy to let someone new in. Like the active transport proteins fighting against a solute gradient, you have to fight against your fears that a girl coming in is only there to gather simps, get attention, and leave with her new paypigs before leaving your group in shambles. See if they're actually a fit – "change your conformation" as matter of speaking, even if it does take energy. As long as they "fit your receptor sites" or show that they have what you're looking for, there's little reason not to let them in. If they don't "fit your receptor site" they don't get in. Simple as that. When in doubt, don't be afraid to use energy and see if someone is worth adding to your group. Being paranoid and ostracizing anyone who comes your way for petty reasons is as easy as it is detrimental
I'm into HEMA (Historical European Martial Arts), and when a joined a group that specifies in my favorite weapon: the Montante, they asked me several questions before letting me in. I had to actually know about the subject. The entire purpose of the group was for discussion of techniques with the weapon, and If I had nothing to add or contribute to that discussion, why would I join? What would they get out of me other than stupid questions I should already know the answer to? There' a lot of people out there who may have joined and made it all into a mess: D&D players who think swords weighed 100 pounds, weeaboos who believe all western swords would be better off if they were folded a billion times and couldn't cut through a bubble bath otherwise, and other people who don't know their stuff could clutter the forum with uneducated questions and theories. You know how it goes: "Too many people have opinions on things they know nothing about. And the more ignorant they are, the more opinions they have." If I may add a corollary to that, it's "… And the more opinions they have, the more belligerent they are about them." That kind of mess would tear our group apart. Gatekeeping was a necessary as it was harmless. I wasn't hazed. I wasn't threatened. I was just asked about what I knew (Did I fit the receptor sites?), and as soon as I proved I had something to contribute, I was let in without any trouble.
Now, does everything need active transport? No.
Diffusion requires no proteins to move particles or use valuable energy. Things like O2, CO2, and steroid hormones can permeate the cell membrane on their own. Water just flows in through channel proteins called aquaporins. Not everything requires energy, and sometimes, when we're wondering why we didn't get into a group while that other guy did, we wonder why. Some people just seem a lot more able to fit in, and those on the inside may see no reason to really question or hassle them. Ton Puey, a Spanish historical fencing master well known to people in HEMA, would probably not even be given any questions (long as he used a verified account), as I did. He's got the credentials. Like a polar molecule entering the membrane, he would easily bypass the bilayer, interacting more easily with the other polar molecules inside the cytosol. Sometimes, the question is up to us: is this something I want? This group or fandom or whatever I want to join, is it really about something I'm genuinely interested in? Am I prepared to face rejection? Is it really worth my time anyway?
In short, I feel like some gatekeeping is in fact necessary. While breathing new life into a group is necessary for longevity and variety, letting anyone in willy-nilly on the other hand can lead to problems down the road. For a lot of us, a bit of "active transport" may be necessary. If we don't just automatically mesh, we need to be aware that we may or may not "fit the receptors." A hard Aesop to swallow is this: no one owes you fellowship. You have no right to dictate other people to like you or want you around. You have no right to tell other people what their criteria should be, nor tell a more knowledgeable person that their benchmarks are unreasonable.
Still though, for those of us on the inside, not everything on the outside is a toxin. Not everything can get in through facilitated diffusion, where someone who already exudes the aura you're looking for stumbles onto your doorstep. That kind of thing is far rarer in real life in comparison to a cell anyway. Sometimes, you need to be willing to expend some energy, commit to that ATP hydrolysis, change conformation, and actively involve yourself in seeing whether or not this person belongs. Most importantly, you need to be willing to let someone new in if they do indeed fit your criteria.
TL;DR: gatekeeping is necessary sometimes. Maybe not all the time, but it does serve its purpose.
