Well, if you really want to know, you can watch the arc to the end since everything will be "explained" there, or you can read the spoiler.
If you really want to know ahead, Darjeeling's comment told you what happened:
The whole reason Blackbeard took the title (Shichibukai) is so he can gain access to Impel Down and recruit some new members. He descended to the last level (6) where the most dangerous criminals are held (for the info, even the existence of this level is unknown to the general public) and recruited the members there. However, he made a competition. He told the prisoners to kill each other in their cells and he will rescue the winners (take them into his crew). At the end, there were 4 prisoners left: Vasco Shot, Avalo Pizarro, Sanjuan Wolf and Catarina Devon. Alongside them, another person joined the crew, the former chief guard of Impel Down, Shiryu (Shiliew).
Anyway, I recommend you watch the anime/read the manga and find out this on your own.