Just like to pop in a word, there's a neat project on, called AGR (short for Artist-Go-Round). In each "round", members take turns adding their unique touch to a starting image, and the member that hosts the round'll finish it. Of course, everyone'll get a chance to design the starting image, and host a round.
Check it out here:
Everyone can join anytime, even while a round is going on, so don't feel guilty about party-crashing! You're invited ...
Despite this project being online/digital, lots of excellent effects can be achieved and enhance the finished painting, so don't hesitate to get out whatever poison you want on it.
If you've any questions, or just want to watch, you can always visit the discussion forum in the abstract and contemporary art forum (even though the project's not necessarily abstract - any styles and media are fine) ... or pm Zmaster, Atena (they're the coordinators) and me ...
Or just post them here, lol!