There is a lot of Grass Roots videos on Youtube.com, you can search them all the way through the 70's and then early 90's and they performed right up until about 2010. Rob Grill had a bad hip and his last couple of years he was on a lot of painkillers. There's one video, probably one of his last and you can tell he is on meds, slurring his words and he had to sit on stool throughout the show, but he still looked youthfull. I believed he still played bass up until about 2003, there's a couple of good videos from the era.
From around 1972 a Medley
Grass Root's Medley - YouTube
from 1989 Temptation Eyes
Rob Grill & The Grass Roots - Temptation Eyes (Live, '89) - YouTube
2003 from PTV, Midnight Confessions
Rob Grill & The Grass Roots - "Midnight Confessions" - YouTube
Tribute to Rob Grill photos and best of music
Rob Grill Tribute - YouTube