Go to the group. 

Go to the tab My Group Images

On the left you now have a menu.

Click to view larger image

Choose the images you wish to add.

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BULK ADD

Tick the images you wish tgo add and then press Add Selected images to Group (see image below)

 

When Uploading a new image

When you upload new artwork you will see any joined groups on the page where you add your description and keywords etc.