When you are asked for help and you are not in a position to do so, you may very gently with a smile on your face say that -
1. Hey can I do this for you later as I am doing my work.
2. I wish I could help you right away but I am stuck with this work, can you wait for me till I finish, I can help you then.
This way you pushing the ball to their court and making very clear that you are willing to help but you struck up with work.