How can I let my subdomain sites managers deal with their own registration?

Hi,
I have a multisite with buddypress installed and just figured out that my subdomains' users would need to register via the main domain registration form and therefore become the main domain's users. What I really want is for the subdomains' site managers to have the option to be totally separate from the main website if they wish to do so. And I might not want their users to be part of the main domain's conversation. So this is an issue...Now having looked through the forums, I found a mention of this plugin: https://wordpress.org/plugins/join-my-multisite/
Does anyone know whether there is an other wordpress or WPMU Dev way? Or is Join My Multisite the only (easy) option to have the subdomains deal with their own registration process?
Thank you in advance,
Betty