I'm new to group RPs, so I'm probably biased, but 1x1 to me seems like the best way. First, if there are any mistakes, then it doesn't feel like calling an individual out in front of everyone else. It also neither seems like gang-banging (if others were to take part) nor confusing as multiple people give their own opinion. Now, I can see some benefits of group RPs, because the mentored individual can see what works and what doesn't outside of a "vacuum", but who says that they can have only one mentor? If you believe group is the best, then is there a limit to how many? Lastly, what if a mentor helped while not being involved in the actual story, whether in OOC or as an editor for their student (to show how good the student's post could be)?