Sub-users do not have a separate login/account in the membership portal.

So you need to log in to the Sub-Users computer to download the installer file. You can do it using a Remote Computer connection if the sub-users are far. Use Remote-Desktop connection/AnyDesk/TeamViewer etc.

Once the software is installed, provide a specific license key to the sub-user. The Agency upgrade provides extra licenses to use on extra devices, you have to allot a license from those for a specific sub-user.

And then next, you have to create the sub-users account on the Explaindio Library website, below is the article explaining how to do this:

Once you have created the sub-users account with a valid email address, register the software from settings using the sub-users account credentials.