Click on the "category" icon on the top left of the chat screen and using [ + Create a new Category ], you can create new categories and organize your chats.

 

From the [ Create a new Category ] screen, put in your desired "category name," put a check mark next to the group chats you want in that category, then click on the [ Create ] button.

The category you created can be chosen via the "category" icon on the chat list.