Is User Sync just a one way synchronization system?

I got the sub site to sync with the master site, but how do I have a two-way sync? How do I get the master site to automatically sync with the sub site? Meaning, when a user registers on a sub site, make it appear on the master site. If this is just a one-way sync system, you guys should make it clear on the plugin's page because it's pretty much useless for me if this is only a one-way thing.