I have a question about registration of multisite!

I hope you having a good day.
I have a question about multisite environment( my case is subdirectory)
If possible, I want to force every user including subscriber to register in only main site.
I will make COURSEPRESS available to my subdirectory Administrator, but in this case, every visitor can register for my network from any sub sites too.
I want everyone to see the term of use of my network, I want to avoid this, if possible.
Do you have an idea for this?

King Regards