The Second Rule of Brawler's Guild (Season 1) is a Legacy achievement that was obtained after defeating Unguloxx during the first Brawler's Guild season.
The leader of the factional Brawler's Guild will send a letter via-the in game mail system with one [Brawler's Pass] (Alliance / Horde) attached that can be used to invite another player into the guild:
You've gathered quite a following down here at the Brawlpub! I'm quite curious to see what your friends can do.
Here's an invitation to give out. Let's get some new blood in here!