There's no such thing as being too busy or not having time to do something. It all boils down to the concept of priorities.
You say no to one task because you choose to do something else during a particular moment.
For example, right now it's 2:43 AM and I'm sitting in front of my laptop writing this article instead of sleeping.
Am I too busy to sleep?
No. I'm just choosing to write instead of sleeping at this moment. I'll sleep when I'm done with this article.
That's my priority right now.
This explains why some people always say that they're too busy to do something.
If you want someone to do something for you, you better work on securing a high spot on their priority list. That's how they won't ever be "too busy" for you.