How to add 2 other computers to my local wi-fi network?

Manage the Wi-Fi connection on your Mac - Apple Support

Connect to the internet with your Mac - Apple Support

pwwwayne wrote:

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???.

Set up file sharing on Mac - Apple Support

Use iCloud File Sharing to share folders and documents with ...

Set the protocol for file sharing on Mac - Apple Support

Connect your Mac to shared computers and servers - Apple ...

pwwwayne wrote:

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???.

Those links answer your question. You have now changed the question to one of file sharing not "How to add 2 other computers to my local wi-fi network". Follow Leroy's advice.