If you are creating an account for a Private Org, enter that Org's ID. Otherwise, if the ID you enter is not taken by anyone else, it's yours, but it's PUBLIC (until you contact us to make it Private)! You may give the ID to whomever you care to, and they may join your org. By doing so, each of you sees the location of the other member(s) of the org. Your Org ID can be up to 40 characters long.
To switch Orgs, hit the ☰ menu icon > Organization > hit a link under 'Org ID'.