Seems like they talk about similar things to us, but aren't quite as "deep". Sorry, though, can't (won't) use Facebook for such things. Sadly they don't seem too active. I'm just seeing some older posts from (most recent) 2020 and back...
Definitely invite them to come to a better place (here)!
Yes, The Subterranean Express - Inner Earth seems to be rather inactive. But I saw two or three things there that I hadn't seen before, so I decided to pass it along.
It's not a matter of joining somewhere else, but there are about three big and rather active groups on Facebook, and they are worth keeping an eye on. (I do it, any other volunteers?). The Facebook groups are huge, too. Gary Fleck's has 13.08 members.
Gary Fleck's group, Hollow Earth, is one: Hollow Earth | Facebook
And then there is Hollow Earth Connection: Redirecting...
And from Europe, The Inner Earth Society, which has gotten a bit wobbly: Redirecting...
No, they're not so "deep" or scientific about things. They never get into discussions about gravity, for example. But they find things that we don't, and we have threads about things that they don't have.
Some of them come over here and peruse the Latest page.
Anyway, there does seem to be some awareness out there.