Multiple subdomains users, but one blog with single pages?

Sorry for the confusing title, but I will try and expand what I want to do:

I want volunteer groups to be able to have their own single page, in the form
group1.mysite.org
anothergroup.mysite.org
etc, with the main site at https://www.mysite.org (not the actual site!)
There’s about 100 of these groups without their own sites, and each one just needs to be able to put their news, or a call for volunteers etc, on a single page, but “wrapped in” the main theme for the overall site, so all they have to worry about is basic text editing.
So, when the group/user logged in, they’d get to see and access and edit only that page. Something’s telling me this SHOULD be quite easily do-able, but I’ve turned up lots of “multiple sites from one install” type of answers, but nothing on this. As a subdomain is simply an .htaccess redirect, shouldn’t that just work “out of the box?”. ie: if, group1.mysite.org was actually mapped to the page https://www.mysite.org/group1 ? But what happens is I create the subdomain in cpanel, it creates a subdirectory with the same name as my test post, I then delete the subdirectory, and end up with a 500 internal server error. In other words:

subdirectory test.mysite.com points to https://www.mysite.com/test
This is actually a permalink to a post, and when I say “new subdirectory called test, it also makes an actual physical subdir, which redirects fine, but of course, to an empty dir. So, I remove the directory, and https://www.mysite.com/test works fine, but test.mysite.com pointing to https://www.mysite.com/test goes 500.