How-To add main site's users to sub-sites?

Hi,

When installing the plugin, I thought it was to synchronise the users between the main site and the sub sites. But when I launch "Synch all site now" (meaning I already defined the main site and added the subsite to it with the Mastersite key) I don't have the users from the main site in the other sub site. It's important for me as one of the subsite is dedicated to support.

How to do it properly?