Designing collaboratively can pose a great challenge, if you don't do it right. From my experience it works really well, when files are passed on chronologically between designers. That way we get our own space and time to work on it in our secret chamber. After some chamber time we give it back into the pool where the next person can take it into their chamber and add some more magic to the open work file. For sharing the workflow I've used cloud structures like dropbox and owncloud. It has worked really well so far, because I do believe in the intelligence of the crowd and that by working in a team talking, doodling and joking (very important!) you get farther because you help each other take the next step up, up, up on the "design ladder". It can be really freeing and inspiring to play around with someone else's ideas and thoughts. For some of us it requires a hidden space where we don't feel judged by anyone when trying out different directions. Other times, when there is a very relaxed atmosphere so we feel very comfortable around our project partners or team mates, collective doodling and live-designing sessions (I recommend no longer than 20min!) can be very fruitful and efficient, as well. I worked with other creatives in that manner creating the OSCEdays, Thinkfarm and OPENMEDiAID designs.