Massively subscribe users at once ? (≈ 100+)

You can do that using mmctl remotely, the Mattermost sysadmin just needs to allow user access tokens temporarily, but the scripts I created for that take care of that too.
There are two different ways, one using a simple mmctl user create approach and one loading the contents of a CSV into the bulk importer utility, see the two scripts for possible ways to achieve that: