I need to assign a default role to new users on a multisite.

I am running a multisite. When a new user signs up, their site and user is created under my network admin. They are given the role of administrator. I need to have that changed by default to a different role.

Let me explain the steps:

1 .- I have a site (MU) where users can register new sites. http://mainsite/
2 .- A user "JohnDoe" can create a new site. http://mainsite/Site2
3 .- JohnDoe is, by default, the site2 admin

What I want is that JohnDoe is not an administrator of site2, only a Author or Subscriber (or another role that I've created).

There is a previous forum thread on this:
https://premium.wpmudev.org/forums/topic/change-the-default-user-role-of-new-sites-multisite#post-71720

Thanks in advance.