I'm pretty sure it's free if you host Matrix protocol server itself:
link And then people connect to it using the Element client app or any other Matrix protocol client:
link.
If you click on the "Download" button above the "Free up to 200 users" line in the link you posted it says "
Element Server Suite"
But we do not need the "Server Suite"... right?
Only the fancy "Suite" is priced there.
If you use the Matrix server over SSH Linux terminal it's free. (I prefer the terminal anyway)
Open Source projects often do this.
They are free... but they do not tell you they are free on the front page.
On the front page they offer you a paid service.
And if you are a business you want to get the paid one.... cuz government and regulation and shite.
But if you go to the GitHub you can just download the binaries directly.
Edit:
Project GitHub:
https://github.com/matrix-org/synapse/^ this is where you get it for free
Edit2:
I took a quick look at the documentation.
And I think we could use this:
link to only allow registration from the tracker members (as you were mentioning before).