1. Wrong: Will you come with us David and Nancy?
Better: Will you come with us, David and Nancy?
Explanation: speaking directly to David and Nancy, we should separate their names from the rest of the sentence with commas. Grammatically speaking, David and Nancy aren't connected to the rest of the sentence.
2. Wrong: Mark and Chris what was the name of the building where you got married?
Better: Mark and Chris, what was the name of the building where you got married?
Explanation: speaking directly to Mark and Chris, we separate their names from the rest of the sentence.
3. Wrong: Tuukka come away from that bear!
Better: Tuukka, come away from that bear!
Also better: Tuukka! Come away from that bear!
Explanation: The name can be separated with a comma, or if the speaker is yelling (as one might be in this instance) with an exclamation mark.