Niantic has added a new feature to Pokemon GO. Players can now create custom Tags that allow them to group together Pokemon. You can access this list of Pokemon from the left side of the game's menu. You can create up to 100 different Tags using the colors Blue, Green, Purple, Yellow, Red, Orange, Grey and Black. You can also check a Pokémon's tag from their status screen, and check all other Pokemon with that tag.
Tag, you're it!