Somehow, people got the idea that this was a group full of of telekinesis guys (telekinesians?). They had spoon benders, levitationists, and all sorts of psychic healers applying for membership. Over and over, they had to patiently explain that they were a group of philosophers—idealists, to be exact. The ESP folks left pretty quickly once they heard that.
Happily, though, a number of philosophical idealists also wanted to join. They had their differences, of course—they were philosophers, after all. But the group was united in its belief that the mind is primary or fundamental. Berkeley was in good company.
Okay, Plato was not as extreme in his idealism as Berkeley. But who is? Plato does not deny altogether that matter exists. But matter is decidedly inferior in his view—a kind of prison that keeps us from realizing our true nature.
For Plato, it's the mind—the psyche—that represents who we really are. And what does the mind properly strive to do? Why, to know the Ideas or the Forms, of course. Plato's Forms are eternal, immaterial objects—like the Beautiful itself, the Good itself, and so forth—that represent the essence of the world and the true objects of knowledge.
Timeless, invisible essences constitute the true nature of reality? Come to think of it, Plato's version of idealism is pretty darn extreme.
Plato is old, but Anaxagoras is ancient. Even in Plato's day, this dude was already part of the distant past. Anaxagoras is best known for being the first Western philosopher to assign a key role to the notion of nous. And nous means—you guessed it—mind, though more in the sense of "intelligence" or "intellect in the abstract" than in the sense of an individual psyche.
Anaxagoras famously claims that there is nous in all phenomena, that it is the "finest and purest" entity there is, and that it somehow orders all things (source). Of course, no one quite knows what any of that means. But we can imagine that Berkeley would be nodding his head in agreement with Anaxagoras's words.
Plotinus is the best known of the Neo-Platonists, a group of philosophers who wrote beginning in the 3rd century C.E. Notice how a lot of the guys in Berkeley's group are from 2000 years ago or more? That shows how he's got old school cred.
Like Anaxagoras, Plotinus emphasizes the centrality of nous. For Plotinus, though, intellect is seen as an image of the One—the infinite, ultimate source of everything. That's right—his view sounds a lot like Berkeley's fundamental claim that all phenomena are just ideas in the mind of God.
It's true that Kant thinks Berkeley is too "subjective" in his idealism. Berkeley sees the individual, empirical mind as fundamental instead of focusing on the mind in Kant's transcendental sense—as that which establishes conditions of the possibility for phenomena to appear. If you have no idea what that last sentence means, at least you're being honest. You may have to go read some Kant.
(We'll give you a hint: Kant is interested in the laws and processes of cognition itself, since it is these laws and processes that allow us to perceive anything in the first place.)
Still, Kant is happy to be a member of the club. Underneath all that fancy language, he is saying that the mind is what structures reality. And he knows that in this belief, he has a lot more in common with Berkeley than with the vast majority of philosophers in the Western tradition.
It's kind of hard to find an idealist in the contemporary world of philosophy, at least in the English-speaking part of that world. Almost everyone there is a physicalist of one variety or another. Far from saying that mind is fundamental, the usual view these days is that mind doesn't even exist.
Nagel is one exception. Don't get the wrong idea, though: it's not like he's a rabid idealist, either—not by a long shot. He would never deny that matter exists. But he has made a career of insisting on the existence of consciousness, and of insisting that consciousness is incapable of being explained by the physical sciences.
More recently, Nagel has gone even further, saying that an adequate explanation of the cosmos has to appeal to the mind—which means that purely materialist theories are necessarily incomplete (source). He has gotten a lot of grief for his claims, but Berkeley would be proud.