I tried to follow reverendj1's instructions (See above? Below? Every time I refresh this page, the articles swap positions), and they didn't work for me on Ubuntu 12.04.1. I had two issues:
1) Simply booting into recovery mode apparently mounts your root partition as read-only, so you can't modify any system files. Another page addresses this, you need to execute this command from the root prompt in order to obtain write privileges:
mount -o remount /dev/[your root partition]
2) But even after I had write privileges, the useradd -G command wasn't working for me. When I tried to execute it, the system would just return the help screen for the useradd command. I don't know why. But I decided to use usermod -aG instead, and that worked. I added my user name to each of the 16 groups in reverendj1's instructions, one at a time. I was being conservative, I saw some error messages I didn't understand when I was experimenting with useradd. Afterwards, I looked at the man page for usermod, and it looks like you could just list the 16 groups all at once, tacking on the user name at the end.
Here's how you can double-check that you did everything correctly. Compare the output of the following command, before and after you make changes:
You should see that your user name has been added to the groups you specified.