Harry sends three copies of a letter to his godfather, Sirius Black, and his two best friends, Ron and Hermione: "I've just been attacked by Dementors and I might be expelled from Hogwarts. I want to know what's going on and when I'm going to get out of here" (3.1).
Harry waits, worrying, "What if they ruled against him? What if he was expelled and his wand was snapped in half? What would he do, where would he go?" (3.16).
Finally, after four nights of no news from the wizarding world, the Dursleys tell Harry they're going out.
Harry is so depressed that he just lies in bed looking at the ceiling.
That is, until he hears a loud cash from downstairs.
He sneaks downstairs with his wand and finds a group of nine people waiting for him.
There is Remus Lupin (see Book Two), Alastor Moody (see Book Four), Dedalus Diggle (see Book One), Nymphadora Tonks, Kingsley Shacklebot, Elphias Doge, Emmeline Vance, Sturgis Podmore, and Hestia Jones (all new friends!).
They're all there to escort Harry out of the Dursley house.
Tonks helps Harry pack up his things. Tonks is a Metamorphmagus, which means that she can change her shape at will.
Tonks is also an Auror, a Dark wizard hunter, along with her boss, Kingsley Shacklebot.
Moody is the leader of the group, and he is extremely paranoid that Voldemort might attack while they're traveling.
He insists that they all travel on brooms in close formation.
Moody also casts an invisibility spell on Harry.
They all fly off from Surrey to London.
Harry is relieved to be leaving the Dursleys, but this flight is also long, cold, and unpleasant.
Harry is glad to get to London at last.
They land on a dirty, dark Muggle street.
Moody hands Harry a bit of parchment to memorize.
It says, "The Headquarters of the Order of the Phoenix may be found at Number Twelve, Grimmauld Place, London."