Easily adding multiple existing users to a multisite site

I manage a multisite network for a school. The sites are private (password protected), but users within the network should be allowed to access other sites as subscribers. (i.e. all students in a class should have subscriber access to the site of their classmates).

When you create a new site or new user, by default only the site admin has access to his site, other users are not added. You have to manually add other users as subscribers. This is ok when you have only a couple sites/users, but as soon as you have 10+ it starts taking a lot of time to add each user manually to each site.

I found many plugins, but they either seem to be very complicated with many roles configurations to do, or be simple but outdated.

This is basically a duplicate of this question: How to add multiple existing users to a multisite site? but like I said, the plugin suggested here seems to be too outdated to use now.

I am basically looking for a plugin to allow me to select multiple existing users on the network when adding them as subscriber to a site, instead of having to type each user name manually and clicking add.

Any easy way to do this? Or known older easy plugin known to work fine under newer WP versions?

Thanks