I'm sorry, I don't see how either of these sites solve my problem.
I'm not trying to add iPad or iPhone to my network; they remain connected.
I wanted to add a MacBook Air to my MacBook Pro so that that I can access the Desktop reciprocally to each other.
I have been able to do this via the Finder dropdown menu under Go<Computer<Network<Share Screen sequence but both Macs disappeared from that sequence a few days ago.
Mysteriously, both computers names have re-appeared "spontaneously" (MacBookAir & MacBook Pro) without me taking a particular action whatsoever!!
I also have a Retina MacBook Pro that I have never been able to add to this Wi-Fi Network???.