You can use userdel username to delete the user's information from the passwd and group files (along with their shadow information). You can use userdel -r username to delete the user's home directory (and the files it contains) as well.
Adding a user to root's group doesn't give them all of root's rights. For example, I can cd to /root but I can't list the files in /root or delete any of them (directory and files all have permissions of 000 for anyone except root) as a user in group 0. At least, I couldn't just now when I tried it...