How do you get more of your best customers?

Hi everyone,
I'm the marketing manager at Rethink Software (https://rethinksoftware.com), where we are building Rethink Files -- the file browser that unifies all your cloud files and drives.
We are currently in public beta and have 6,000+ users in our system. Now the next challenge is to understand who are our most engaged users and how we reach more of them. I would love to hear your experience on how you have or would tackle this. Thanks!