I thought about this, too, and he's not with the Sesame Street gang in The Muppets Take Manhattan, The Muppets: A Celebration of 30 Years (despite Grover being a waiter at that event), or A Muppet Family Christmas (I'll cut his absence from The Mupets Celebrate Jim Henson a little slack since the finale didn't seem to try to include as big a group of Muppets as those other situations).
It seems like Mr. Johnson didn't start to become a major (or semi-major) character until the 1990s. It seems like he wasn't in many 1970s sketches, either (though the puppet was uses in quite a few sketches without Grover, though most likely not meant to be the same). I don't know whetehr the poetic memory sketch is from the late 1970s or early 1980s, but other than that the only segments I know of from the 1980s are Grover the singing telegram deliverer, Grover the singing and dancing waiter, and the airport resturaunt sketch (and that one I'm mostly just guessing that it's not from the 1990s).
It seems like it wasn't until the 1990s when Mr. Johnson would have the misfortune of having Grover serve him almost everywhere, besides a resturaunt, and also then when it was common for him to even go to other resturaunts (besides Charlie's) only to have Grover as his waiter. Then in the early 2000s he wasn't seen often, and then seemed to have something of a comeback in 2004, and has made at least one new appearance every season since.