It was decided months ago - years, really.

'There was silence in the room, broken only by gentle thuds, as Hermione continued to throw books on to one pile or the other. Ron sat watching her, and Harry looked for one to the other, unable to say anything. The measures they have taken to protect their families made him realise, more than anything else could have done, that they really were going to come with him and that they knew exactly how dangerous that would be. He wanted to tell them what that meant to him, but he simply could not find words important enough.'

Extract from Harry Potter and the Deathly Hallows, J.K.Rowling