@Hyolobrika@mstdn.io usenet essentially is people agreeing to exchange posts with each other. which groups they exchange is decided by each admin. much like fedi, except that you have to explicitly add servers from which you accept. those also send you posts from other servers that you don't have, so you don't need to setup a feed with everyone.

the website of big-8 has a wiki with some stuff about the history etc. https://www.big-8.org/wiki/How_the_Management_of_the_Big-8_Developed

tl;dr you are free to create new groups in any hierarchy except big 8. if anyone does accept them is on another paper.