Brawler's Guild is finally back for Legion, and to get invited you need a Blood-Soaked Invitation. If you were previously a member, you still need a new invitation for the Legion version, so everyone needs to get one.
The invitation can drop from any elite mob in Sheild's Rest in Stormheim. It has a pretty high drop chance, you you don't really need to farm for it for a long time (around 2 % drop chance per kill). If you are having problems with the mobs, join a group that is farming for invitations from the Dungeon Finder.
There are also other sources for the invitation, such as randomly from mythic+ chests.