The Them are Adam Young's gang in the little village of Lower Tadfield. Their name derives from the fact that they don't actually have a permanent name, since Adam renames the gang every few weeks according to whatever his latest enthusiasm is or his latest reading has been. The locals simply refer to the group darkly as Them, so this has become the default name.
The Gang's base of operations is in a weed-filled hollow furnished with old milk-crates and bits of wood, of the sort beloved by adventurous pre-teens. They consider it a "secret base", but it's a fair bet that their parents know where it is.
As noted on the Adam Young page, the Them are in part a reflection of William Brown's Outlaws, both in their general attitude (meeting in a "secret" place, harmless mischief, new games following the fads and fancies of their leader) and the fact that they have an ongoing feud with a rival gang led by and named after a personal enemy of the leader (Hubert Lane and his Laneites in William, Greasy Johnson and his Johnsonites in Good Omens). The difference comes in the standard of characterisation.
Now I know I'm going to put some hackles up here, granted Richmal Crompton did create some very memorable characters, William himself, of course, plus some of the adults who tried to handle him and of course the dreadful Violet Elizabeth Bott. However, many of her characters were just cyphers, and sadly this includes the outlaws (most of the time at any rate). Douglas was the oldest (12) and the tallest, Ginger was the one with red hair and Henry was the one who wasn't Douglas or Ginger. That's about it really, can anyone else remember anything more about them? Furthermore Joan, the girl next door, the only girl to ever count as a member (Violet Elizabeth doesn't count, her own opinion notwithstanding), was a standard girly girl. She was naive enough to follow William devotedly in practically anything and so pretty that when she, in her best party dress, helped William, in Father Christmas disguise, deliver presents to a poorer child, the little child immediately mistook Joan for a Christmas Fairy and asked in awe-struck tones if she might touch her. Sweet, but it wouldn't appeal to most modern girls!
Pratchett, on the other hand, characterises even his supporting cast. Yes, the Them are all devoted to Adam, but brainy Wensleydale, grimy Brian and tomboy Pepper are each more memorable than William's gang, even with only one book to their names.