Thanks a lot for your suggestion. I have asked one of them to write a letter for me. But I have helped a lot, so I want to emphasize that in contribution.
Technically, if you are on the author list, you would've already claimed the authorship for those 'help'. Your boss must be a big shot! :) Anyway, recommendation letters might be the best form. Those letters can at least help you claim that your research has impacted a lot of others' work...
Actually, no. Their paper is not out yet. And I want to save it for RFE material. My boss is a OK big shot. :) I don't have extra words for more recommendation letter. So I guess I will just use the one I have now. Thank you for your advice.