This is what option groups are specifically designed to do; if you want to be able to select multiple checkboxes on a form, they can't be in an option group; they must be added straight onto the form.
You can always make them look the same by putting a box around them and adding a label.
Just an added thought that might be helpful:
An option group is designed to deposit one piece of data into one field (whichever of the options you choose). Separte individual check boxes will enable your user to select multiple choices but each check box will require it's own field.
