A pirate could choose to join the Shichibukai if a space became vacant (which means that at least one Shichibukai must resign or be expelled from the group, the former being Jinbe's case and the latter being Crocodile's case). However, in order for one to have become a Shichibukai once this position opened up, one must have been able to exhibit their strength against other pirates while choosing to pledge loyalty to the World Government (this ironically encourages pirates to gain infamy to become a Shichibukai) by making a pact with the Five Elders. If the pirate in question could be used as intimidation, the position may be offered to them, if no other pirate has accepted the position or proven to be more of an asset to the World Government.
As expected, most members were already notorious pirates with high bounties, but even relatively unknown pirates could have been admitted if they accomplish a feat to prove their strength. Blackbeard's victory over Portgas D. Ace and Trafalgar Lawobtaining and sending the hearts of 100 pirates to Marine Headquarters are examples. The World Government can also be extorted by pirates into offering the prestigious status. This was demonstrated by Donquixote Doflamingo, when he ransomed the heavenly tribute meant for the World Nobles to be appointed to a vacant position.
Some cases also happened such as Jinbe, Buggy, and Ace, who were offered a position in the organization when the World Government's Carrier Bat sent a letter to their location and the pirates reply by coming to Marine Headquarters as a sign of accepting the invitation or declining the offer. According to Chew, refusing to accept the invitation would cause the chosen pirate's bounty to rise even higher