Nevertheless, if everyone is in the same group, then you need to set your default group to represent that.
You'll also need to change the group of any files you've already created.
hadoop dfs chgrp -R groupname /
Next, to allow multiple users to run mapreduce jobs, you'll need to set your configuration directory to be a place that is accessible to ALL the boxes. I'm using an nfs mount, but you could use hdfs just as easily.
Make sure that directory exists and is writable by the group mentioned above (or at least that all your mapreduce users can write to it).
At this point, that is all that I know needs to be changed. I have two people (including myself) using our hadoop cluster. So, I'll let you know as we run into more problems.