Unfortunately, there's no option to add Robux to your group directly, but here's how you can easily circumvent this and ultimately donate Robux through group funds:

1. Head to the “Store” page.
2. Select the "Create or manage group items." option.
3. Click on the "Game Passes" on the left side, then fill out the necessary details, add an image, and upload it.
4. Set a price and then save it.
5. Return to group creations, and you'll notice your dummy group game pass.