You don't see ALL the other computers that you are sharing a public network with because, generally speaking, a computer has to have some sort of Sharing turned on to show up on the list.
Your Mac will show up on the SHARED list of other users if you have either File Sharing or Screen Sharing turned on in the Sharing panel of System Preferences. There are perfectly valid reasons to use File and Screen Sharing, but if you don't use them you should make sure that they are turned off, especially if you use your Mac at a public Wi-Fi hotspot. If you do use File or Screen Sharing on your Mac, check out this tutorial from Apple to make sure you are only sharing what you want to share and not more.
Here's how to check the status of File and Screen Sharing on your Mac:
- Go to System Preferences from the Dock or the Apple Menu.
- Go to the Sharing pane.
- On the left pane is a list of all the things on your Mac you can share with checkboxes. If File or Screen Sharing have checks next to them you are sharing.
If you see other computers in your SHARED section when you are on your own home network, you need to check them out.
If the other Macs and PCs in your home show up on the list, you might want to check them out to find out WHY they are sharing info and turn off sharing if you don't need it.
If you don't recognize a name on the list, the first thing to check is to make sure that your own Wi-Fi hotspot is secured. If it isn't, anyone within range can connect to your network. (Check out this tip for info about home Wi-Fi security.)
If your Wi-Fi is secured and you still have a computer on your SHARED section that you don't recognize, it might be an internet-connected device in your home like a TiVO, TV, BluRay player or music player. These devices often have default names that you never set and have never seen. I have several such devices on my home network and one of them shows up as SHARED.