To add images to an image group you will first need to have an album setup. To see how to do this click here. Click on the album you wish to add images from.
The album will display the images currently in it and below that you will see a link that states 'Add Pictures to an Image Group,' click on that link.
The screen will then change slightly and each image will have a checkbox, select which images you would like to add to a group. At the bottom of the page you will see a drop drop box that shows all the groups that you can add your images to. From this list select the group that you wish to upload your images to then click the 'add images' button.