Main blog vs Sub blog users

Hi,

I'm new to WPMU and am finding the user registrations confusing. I have tried searching the forum and am more confused. So, I'll ask my question here.

1. When a user signs up on the main blog as a user, they are added as a main blog subscriber, yes?

2. When a user signs up for a user account on a sub blog, are they also added as a main blog subscriber?

3. I've read the post here:https://premium.wpmudev.org/forums/topic/adding-all-new-users-to-main-blog#post-12410
Does this mean that if I create a blog for an existing main blog user, they are no longer a main blog subscriber?

I'm a bit confused. Essentially, here's what I want to happen.

- Anyone who signs up on the main blog are added as users of the main blog.
- Anyone who signs up on the sub blog are added as users of both the sub blog AND the main blog.

Does WPMU currently work this way?

~ Lina.