the MyBookWorld runs busybox, which is sort of linux, sort of not. It has some command differences from your standard vanilla linux distro. For help with some commands for busybox, just type busybox from the cmd line, it will show a short help file.
My apologies, I thought that I had a clever answer for you here. On further inspection, I can't find an easy way to do this on the MBWE. Kevku may be on the right track, editing the /etc/passwd file may be the only way to do this manually. Please remember that editing this system file can be dangerous and could render user accounts, or your whole system unusable if you mess anything up.
That being said, I think you could successfully do this if you know how to go about it. The following page explains how the /etc/passwd file is structured. Then, it's just a matter of looking at /etc/group, finding the number of the group you want to change to, and edit the user's line in /etc/passwd to reflect the new number.