Virtualbox Ubuntu 12.04 admin privilegs problem

I have Ubuntu 12.04 Virtualbox guest in Win7 

I get admin privilegs problem after executing
sudo usermod -G vboxsf jack

I dont had admin(sudo) privilegs anymore and to fix it I found solution, which needed some improvements

1. boot to recovery mode during Ubuntu Virtualbox startup holding down SHIFT key
2. login as root and execute
3. mount -o remount,rw /
4. usermod -a -G sudo jack
5. reboot