This post sums it up pretty well:
https://hive.blog/steem-engine/@steem-eng/project-spotlight-banjo-token
And this is the original announcement post:
https://hive.blog/steemspeak/@inertia/introducing-banjo
Back then, Banjo was exclusive to SteemSpeak. Now the invite is public (which is shown on the @banjo account banner).
Thanks. I'm amazed I didn't know about this.