A short wizard wearing a purple porkpie hat attended Bill and Fleur’s wedding. During the reception he sat with Hagrid and Charlie singing ‘Odo the Hero.’ (DH8)
Because of the purple hat, the general demeanor, and being a member of the Order of the Pheonix and therefore a likely guest at the wedding, we might guess that this wizard is Dadelus Diggle. However, Diggle is supposed to be somewhere guarding the Dursleys. Plus, Harry knows him and wouldn't be likely just to identify him as a "squat wizard in a purple porkpie hat."
What is a porkpie hat? Here's a photo:
From the Web
Porkpie hat from Wikipedia