You've missed the point. Drop.io isn't Team collaboration software. This application would be focused on helping teams collaborate, drop.io is (based on what I saw) more focused on file sharing collaboration.
I really think that it should not matter so much with this idea that there are similar existing startups. Since most of the time the actual product changes anyway, it is important that you have a good direction. I would be interested to see how many users Drop.io and Yammer have. That could actually be a validation that you have users that desire what you want to build.