"I'll go," she said, smiling. "I'll go," she said smiling. Is there a rule for when a comma should/should not be there?
The first one is right in that case. With the comma. (But if it were "with a smile" you wouldn't need one).
i agree about the first being correct... imo, 'with a smile' could also use a comma before it, though it may not be mandatory...
It's a lot easier to get the hang of punctuation if you read the sentence aloud, since in this example you're using it at a natural pause. When you read the sentence, do you hurry those three last words together? Or do you pause at a certain point? E.g. It's not like that not anymore. Where does the naturally spoken pause come? You can of course substitute other ways of saying the same thing as also suggested, just explaining how it's a bit easier to figure out your original question for future reference.