Can anyone join a Fleet without permission?
No - only the Fleet creator can add/remove Clients from a Fleet.
You can use the Diode Network Explorer's Fleet page to view and manage your Fleets. If you don't have MetaMask setup yet, do that first!
If you've installed the Diode Client, it by default will use Diode's Developer Fleet - if you want to use your own Fleet, you can add your Client to it after you've created it.