OK, let's see how far people can go. I will start with a show, name a guest star and then name the show he/she was a regular on. Take that show, name a guest star and then name the show he/she was a regular on. Once a show or actor is used, they are out of play.

Brady Bunch
guest star on the Brady Bunch: Davy Jones (Getting Davy Jones)
regular on: The Monkees
guest star on the Monkees: Vic Tayback (Son of a Gypsy)
regular on: Alice

So out play: Davy Jones, Vic Tayback, the Monkees, Alice, and the Brady Bunch.

Now name someone who guest starred on Alice and a show they were regulars in

Guest star on Alice: Sorrell Booke
Regular on: The Dukes of Hazzard

[By the way, Vic Tayback was also in The Monkees episode "For Pete's Sake."]

Guest Star on: the Dukes of Hazards: Dick Sargent (Jude Emery and Deputy Daisy Duke)
Regular on: Bewitched

Guest star on Bewitched: Billy Mumy
Regular on LOST IN SPACE

Easy one -

Guest Star Kurt Russell on Lost in Space (The Challenge)
Regular on: The Travels of Jaimie McPheeters

I will even go one more:

Guest Star: Burgess Meredeth (several episodes of Jaime McPheeters)
Regular on Gloria (Played the vet Willard Adams)