How to add a user to main site with Batch Create?

Trying to add a group of users to the main site on my multisite install. What is the site slug of the main site? I've tried leaving it blank, putting "/" and the url of the main site but nothing works.

I saw this thread but it was never clearly answered: https://premium.wpmudev.org/forums/topic/how-to-use-batch-create-to-add-users-to-main-site