In 5.1, the Brawler's Guild is being implemented.
There are only 2 ways, it seems, to get into the guild. One is to win a bid on an invitation from the BMAH.
The second is by invitation from someone already in the Brawler's Guild.
Early on, when this is fresh and new, it may be worth tossing some money at the BMHA invitation and then selling invites through trade chat.
Here is some more info:
After some ongoing discussion between the player base and Blizz, there has been more clarification on how invites will work.
It seems like Blizzard is pretty set on keeping invites VERY limited.
MMO-Champ tossed up some updated info, and one tidbit was specifically targeted at how invites will work.
It seems like that just because one person wins one of the BMAH invites doesn't mean that they can just start inviting all their friends.
According to this tidbit I posted below, you must reach a certain rank in the Brawler's Guild before you can invite people, and even then, you have to EARN invites. Overall, it seems very fated and limited, which may in turn encourage more people to actually sell their invites since they have to work for them, rather than just inviting the entire realm.
About Those Invites
Invites to the Guild will (at least initially) only be available for purchase on the Black Market Auction House. We’ve taken this approach both to control queue times, and also because the Brawler’s Guild is meant to be kind of a small, underground thing, and a way for players on a realm to come together. At first, that will happen because invites will go out from one player to another based on who buys those initial Black Market invites. After reaching a certain rank in the Brawler’s Guild, each of the new members can eventually earn one new invitation of their own to pass out to someone else on the same realm, so the Brawler’s Guilds on each realm should get progressively larger and larger over time. We may also investigate alternate methods of acquiring invitations or making them more plentiful.
>>>After reaching a certain rank in the Brawler’s Guild, each of the new members can eventually earn one new invitation of their own to pass out to someone else on the same realm<<<