Just before this show premiered on 9/20/1978, Robert Urich briefly appeared as Dan Tanna at the end of the "Charlie's Angels" episode of 9/13/1978: Charlie's Angels: Angels in Vegas (1978). Supposedly Dan Tanna was a friend of Charlie and Bosley.
In the second season episode "Classic Connection" pop singer Wayne Newton plays a racing car friend of Dan's named Justin Marsh. During the episode a character remarks how Justin bares a very strong resemblance to "Wayne Newton". In the third season episode "Dead Ringer" he returns to the series this time as himself.