I, and probably other people, predicted a long time ago that Buggy was promoted to Shichibukai when he received that bat messenger before the time skip.
From out point of view it's just whack that someone so weak becomes a Shichibukai, however from the point of view of the WG, the Marines and the world it's certain not abnormal.
1) They don't know Buggy is weak
2) Buggy was deemed one of the two people who caused the ID rising
3) Buggy was part of the crew of the Pirate King
4) He's 'friends' with Shanks, one of the Yonkou.
5) He's leading an entire horde of malicious pirates originating from all levels of ID, meaning several worth over a 100M beri.
6) He caused a ruckus during the war with that army
So yeah to the world he looks like a massive big shot, trained by the Pirate King himself and in the league with one of the Yonkou, controlling a horde of pirates and criminals who, in their own right, were famous and had high bounties, which he broke out of an impenetrable fortress and used it to fight in a war between Marine HQ and Whitebeard and lived to tell the tale. This is pretty much how the world sees him, so it was pretty much a no-brainer for the WG to invite him to the Shichibukai as it provided two major advantages:
1) They got someone who is an 'authority figure' to the world as I just described. It's unclear how well the Marines themselves are aware of Buggy's strength, however in the least they know how the world perceives him.
2) They get instantly a certain amount of control over all the convicts that escaped ID.
It's not really that odd, hence why this was the first thing I suspected when Buggy received that bat messenger.