The driver of the bus takes the passengers back one last time to things left undone on earth

It's called "Dear God" (1996) with Greg Kinnear. At the time the bus crashed and all the people die a baby (Kinnear) was born and the 4 people become his guardian angles and together they answer the Dear God letters in the post office and Kinnear helps the people say goodbye to their loved ones. Excellent movie

Edit: disregard , I was wrong it is heart and souls, I had the two mixed up. "Dear God" has Kinnear answering the dear god letters and "Heart and Souls" has Robert Downy being reunited with his guardian angles to help them say goodbye to their families. Good Job LunarBlue.

Or it could be Heart and Soul with Robery Downey Jr. Pretty much the same plot line.