Greetings, @stephenmeyer3-me! I'm glad to assist you with setting up a subcategory for your group items in QuickBooks Online. Let me guide you through the process to make it super easy.
I'll show you how:
Go to Settings ⚙, then choose Products & services.
Select More ▼, then Manage categories.
Click the CreateNew Category, then enter the name of the category.
Checkbox the This category is a sub-category.
Hit on Save.
For your convenience, visual references are provided below as an additional resource.