If you've got a project ("Host birthday party"), an action group ("Prepare food") can help you organize within that project.
Code:
Host birthday party
- Invite Guests
- Prepare food
- Bake cake
- Buy drinks
- Find birthday candles
- Clean the house
If you find yourself creating related actions in a Single Action List, it might be a hint that it should really be its own project or single action list. But it could also be okay to just leave them in the single action list.
For one person, "Make Cat Toys" and "Buy birdseed" are just two mostly unrelated actions. For someone else, they are both part of the very important "Entertain the cat so she stops destroying my couch" project.
In the end, it's not about following the GTD rules, it's about organizing your stuff in a way that helps you, well, Get Things Done. :)