Considering Bill Weasley was the Secret Keeper for Shell Cottage, how did Harry and Dobby Apparate to the cottage? Or, for that matter, Luna and Ollivander?
There`s three theories going around for this:
1 - Ron has been made into an additional Secret-Keeper for Shell Cottage.
2 - The Fidelius charm wasn't cast until after the trio reached the cottage, since they didn`t know they were in immediate danger yet before that.
3 - Elf magic is beyond Wizard Magic so it didn`t affect Dobby.
The second theory is the most likely, but none are confirmed :)
I think Dobby didn't Apparate into the area protectd by the charm, but a little way off, in the beach. Even the death eaters could hang around the street outside Grimauld Place yeah?
And then when Dobby and the rest Apparated, Bill and Fleur came up didn't they? And then they were told to "come up to the house" weren't they? (Correct me if im wrong:-)). So that means that the Fidelius charm was cast before Harry arrived.
It's because they apparated to the space outside Shell Cottage.
'Hermione?' he said suddenly. 'Where is she?'
'Ron's taken her inside,' said Bill, 'She'll be all right.'
They were all sitting in the living room when he entered the little hall, their attention focused upon Bill, who was talking. The room was light-colored, pretty, with a small fire of driftwood burning brightly in the fireplace. Harry did not want to drop mud upon the carpet, so he stood in the doorway, listening.
Bill placed the Fidelius Charm on the Cottage itself.
“Fidelius Charm. Dad’s Secret-Keeper. And we’ve done it on this cottage too; I’m Secret-Keeper here."
So that didn't stop them from apparating there.
It's most likely a plot hole. Like the many others Rowling has. We try so hard to cover them but this one is pretty obvious. Bill is the secret keeper at Shell Cottage
“How are they protected?” asked Harry. “Fidelius Charm. Dad’s Secret-Keeper. And we’ve done it on this cottage too; I’m Secret-Keeper here.
Even if dobby apparated inside the Fidelius Charm they still wouldn't be able to see it! They would have to be told about it by Bill. The Fidelius Charm is used so many times as deux ex machina that it's ridiculous. When you see post after post discussing the plot holes around it, you know that there are many problems...
I'm surprised that this is even a question--the third reason is clearly the best reason--Dobby is the one apparating in and out of Shell cottage, everyone else is just a tag-along. Dobby's ability to apparate to places where wizards can't is shown repeatedly throughout the series. The most important example is just before this, he goes to Malfoy Manor. The Malfoy's house is protected and no one can apparate in or out, but Dobby does so easily. He also is shown to apparate into and out of Hogwarts, where no one, not even Dumbledore (despite the 6th movie) can do so. He does this in Book 2 and both Kreacher and Dobby apparate within Hogwarts repeatedly in Book 6--again, Wizards cannot do this, house elves can. Also, Kreacher managed to apparate out of Voldemort's cave, which was heavily protected, and no wizard or witch would have been able to do so. Therefore, the Fidelius Charm obviously does not apply to house elves, as this has been proven over and over again in the Harry Potter universe. The answer is number 3, Dobby did it.
If this was house elf magic, then how did Remus reach there. It must have been done before the trio reached, this is the most likely thing to happen.
And also we know that when Ron left the trio midway, he went to Shell Cottage so the greatest possibility of using the charm lies at that point. That is how Ron could say the name and tell Dobby about it. And maybe all Order members knew about the location or Bill told Remus himself at some point and that is how Remus knew it.