Who was Chrono waving to at the wedding?
That's Melchior, one of the three gurus.

You might remember him from the Millenial Fair, where he sells some starter gear at the beginning of the game. Later on, he helps Chrono and the gang to reforge the Masamune. He also plays a part in the Antiquity storyline.

The Chronopedia also mentions his appearance in the ending:


  Melchior appears in the ending after you defeat Lavos. Melchior comes to Crono, and Marle's wedding with a bottle. Crono and Marle notice him because they wave back.