Prior to the events of the Deathly Hallows, Snape confunded Mundungus, on Dumbledore's order, so that the Order used the decoys as a plan to remove Harry from Privet Drive.
“You will have to give Voldemort the correct date of Harry’s departure from his aunt and uncle’s,” said Dumbledore. “Not to do so will raise suspicion, when Voldemort believes you so well informed. However, you must plant the idea of decoys; that, I think, ought to ensure Harry’s safety. Try Confunding Mundungus Fletcher. And Severus, if you are forced to take part in the chase, be sure to act your part convincingly. . . I am counting upon you to remain in Lord Voldemort’s good books as long as possible, or Hogwarts will be left to the mercy of the Carrows. . . ”
Now Snape was head to head with Mundungus in an unfamiliar tavern, Mundungus’s face looking curiously blank, Snape frowning in concentration. “You will suggest to the Order of the Phoenix,” Snape murmured, “that they use decoys. Polyjuice Potion. Identical Potters. It’s the only thing that might work. You will forget that I have suggested this. You will present it as your own idea. You understand?”
“I understand,” murmured Mundungus, his eyes unfocused. . .
(Deathly Hallows, Chapter 33, The prince's tale)
And, of course, Moody and the Order couldn't use their initial plan, as Moody himself explains:
Moody dropped his sacks at his feet and turned to Harry, “As Dedalus probably told you, we had to abandon Plan A. Pius Thicknesse has gone over, which gives us a big problem. He’s made it an imprisonable offense to connect this house to the Floo Network, place a Portkey here, or Apparate in or out. All done int he name of your protection to prevent You-Know-Who getting in at you. Absolutely pointless, seeing as your mother’s charm does that already. What he’s really done is to stop you from getting out of here safely.
“Second problem. You’re underage, which means you’ve still got the Trace on you.”
“The Trace, the Trace!” said Mad-Eye impatiently. “The charm that detects magical activity around under-seventeens, the way the Ministry finds out out about underage magic! If you, or anyone around you, casts a spell to get you out of here, Thicknesse is going to know about it, and so will the Death Eaters.
(Deathly Hallows, Chapter 4, The seven Potters)
But I'm sure the members of the Order could've imagined many other ways (and maybe even some that might have been safer) to get Harry out of there. Just for example (and I'm no Auror nor a member of the Order, so imagine what THEY could have thought to):
using the Polyjuice Potion the other way, so that NO ONE looked like Harry (the answer to why they didn't do so is here, but that does not mean no one in the Order would have thought of it)
letting Harry escape with speed and discretion, disillusioned and flying his Firebolt;
like the Dursleys a few minutes earlier, driving a few miles, then disapparating or using a Portkey
or other possibilities, some of them being discussed there.
So how did Mundungus convinced the other members of the Order to adopt his plan*?
(I was never under the impression that he was much loved nor considered any brilliant mind among the Order.)
*Other than telling them "We need to do so, otherwise there won't be any battle of the Seven Potters" of course!