This was Groucho's final public appearance, and he hadn't been on television since his honorary Academy Award in 1974.
Marx was showing the signs of senility as early as January of 1973, and it's noticeable in his Tonight Show appearance in May of that year. By the time the 1974 Academy Awards show came around, he was very frail and had trouble making a short speech to accept the award. Groucho's son Arthur, who was embroiled in legal battles with his father's live-in companion Erin Fleming (for which litigation continued into the mid 1980's), was cognizant of this, and out of protection for his father, he made it known that Groucho was not accepting any further offers for public appearances.
When "Joys" was being cast, Bob Hope (out of courtesy) offered Marx a role in it. Arthur Marx was opposed to the idea but relented to persuasion by both Johnny Carson (who idolized Marx) and George Burns. Burns offered to do the scene with Groucho. It was said that the shooting of this short scene took quite a while, as Marx was somewhat disoriented and had trouble concentrating. The scene is a compilation of many takes (as is evident by the different sitting positions of Marx and the slightly different positions of his cap in seemingly contiguous time). The patient Burns worked with Groucho until they had enough decent footage to patch together the minute and a half scene, and the applause and laugh track were dubbed over it later.
But even in old age and senility, you can still see some of the old Groucho. When Burns notes that nobody is getting younger, Groucho asks George how hold he is. Burns (who was pushing 80 at the time) states that he's 76. Groucho retorts "You're getting younger." We even get to see a bit of the famous Groucho leer.
To correct another review, the final comment Groucho makes to Billy Barty (who played a miniature version of him in the scene) is "Don't just stand there, mingle; I want to have a good time."
It's a tribute to Marx that he even attempted this, and while it's obvious that this isn't Groucho in his prime (nor should that be expected), it's a darned good effort nonetheless.
You can find this clip on YouTube.
