For Bundled product the sellers add products to the Bundle and also the custom options such as color, size etc. So that the customers can configure the Bundle product by selecting custom options before purchasing.
Whereas for the Grouped product there is no feature as such. The seller simply adds the products to the grouped product and the Grouped product will be available on the front end for the customers.
To make it clear, Grouped products are supposed to unite simple products for customers’ convenience, like the products frequently bought together.
While Bundle products are supposed to let customers built a complex product they want.