Camoa Cup
The first item you need to get Montblanc is the Gigas Pendant, which you can get by winning the Camoa Cup. This is a clan tournament held in Camoa, where you fight in three six-on-six battles. You have to win all three battles using only physical attacks -- any attack which uses Magic Points or MP is forbidden. Once you win first place, you are rewarded with 2400 Gil and the Gigas Pendant.
Fluorgis Cup
You can get the next item you need, the Pin or Order, by winning the Fluorgis Cup. This is another clan tournament, this time held in Fluorgis. It consists of three one-on-one battles, each with different rules -- in the first battle, you must win without using any status buffs; in the second you cannot use any weapons or abilities that use fire, ice or lightning; and in the third, reaction abilities are banned.
The Last Step
The Last Step is the fourth is a series of missions you do for Mack, the White Mage, after the First Step, The Next Step and A Step Further. These are simple trading missions -- all you need to do is bring certain items to Mack. In The Last Step, you trade the Ocktor Tome of Medicine, which you should have acquired while completing the main storyline, for the Earrings of the Dead.
Wanted: Friends, Kupo!
This is the mission where you finally get Montblanc to join your party. Once you have collected the three items above, the mission Wanted: Friends, Kupo! becomes available from the Inn. All you have to do to complete the quest is go to Baptiste Hill and talk to Montblanc and accept his request to join your team.