[Add Existing Users] Add users to multisite


We are trying to implement a use case like this:

1) We will put our subsites into 2 categories. Category 1 are sites for all members across the network to use, which include the Training Center and the video site which every member should automatically get access.

2) Category 2 is for local groups. Each local group will have its own sub site.
3) When a user signs up to any of the subsites, he will automatically gain access to all subsites in Category 1
4) After that, he can still sign up to subsites in Category 2.

For 4) we found a WordPress plugin Network Subsite User Registration (https://wordpress.org/plugins/network-subsite-user-registration/)

Do you have any suggestion as to how we can achieve 3)?

We are looking for something that does what Add Existing User plugin does, except:
1) it needs to be automatic.
2) It will only add a newly registered user to all sites in Category 1.

Any suggestion you can provide is greatly appreciated.