In the top right corner of the Brickplanet website there is a "Create" button. Click it and go to "Ad Campaigns", and from there you can create an add by clicking "Create Ad" button.

To create an ad for your group, first you need credits, I recommend 50 credits and go to the button written create and then go to ad in create then create an image and just create it and that's it your ad has already been made