Grawp was not originally supposed to be Vincent Crabbe's mentor. That title belonged to Fenrir Greyback, but Grawp was chosen after Greyback went missing.
Many doubted Grawp's ability to communicate with Crabbe, considering he is a drooling giant, but -- as was proven in the Draco Malfoy story -- Grawp does pass messages to Crabbe.
That does not impress Draco, however, who thinks to himself:
"You’d pitied Crabbe when it was announced that Grawp would be replacing Fenrir Greyback as his mentor, and you pity him even more now for actually taking that brainless beast’s advice to heart."