Though not common, there have been round robins in comics. Before they start their round robin, the people involved have to agree on some guidelines for their round robin "story." These can be, but are not necessarily limited to: amount contributed, plot, characters, style, continuity, goals, what to do about was previously created, time constraints, things not allowed, specific rules, etc.
To be more interesting, usually comic round robins have different writers - sometimes the writers are also the artists. Sometimes they have different artists, especially if they want the "look at all the people involved" sort of effect. With three or more writers, there is the question of whether a writer somewhere in the middle can only read the last entry (and since they don't have access to the previous entries, the story becomes very "schizophrenic") or all the work previously made (making for a more coherent story). It is sometimes the duty for the last writer with the overwhelming task of "wrapping things up," or everyone involved can get together to finish the project, or it can just be left hanging, maybe for someone else to continue. Also everyone is usually given a chance to see, and possibly have a good laugh over, the "completed" work.
Here are some examples of Round Robins in comics:
DC Challenge: a 12 part round robin in the DC universe with 12 different writer/artist teams. Each were required to make a cliffhanger for the next team to solve and there was no writer collaboration, except in the last issue, where it was wrapped up.
3 Word Game (In the Comicvine forum under RPG's): Well, okay this isn't a comic. I put this on to show an example of a round robin, for readers who don't have any round robin comic issues. This is a story where anyone can add three words to it. They have the opportunity to read all the previous entries (whether they do or not...) and to post another 3 words, if they desire. Probably not very cohesive. At least for now, it seems never ending.