When you are a part of multiple organizations, your user account is separate for each organization. This allows for more granular control of user accounts by the organization admins, such as requiring two factor authentication. The organization short name is used to specify the org you are logging into. Once logged in, if you have verified your email, you can change between all organizations you are a part of.
Organization short name
If you are a part of two or more organizations, when you log into Command you will need to specify the organizations short name to indicate which org you are logging into. This short name is a unique identifier for the org. When logging in, if you do not know the short name, you can select the Email me a list button to have them emailed to you, along with email verification links for each org.
Changing between organizations
You can see all of the organizations you are logged into by selecting the organization name on the left sidebar in Command, and move between organizations from that list.
Select any of the orgs listed to switch into them
If the org you are switching to has higher authentication requirements, two factor authentication or SSO, you will be required to log in to that account when changing to it.
Unable to see organizations you are a part of
You may find that not all of the organizations you belong to show up in the list when selecting the org name in Command's left sidebar. This will happen when your email is not verified in each of the organizations.
The easiest way to do this, especially when you are a part of many organizations is go to the login screen and select the Email me a list option as shown in the above Organization short name section. This will send an email with verification links for every org you are a part of.
The other option is to log in to each organization, and select the Verify Email button, which will send you an email to verify your email for that particular organization. This will need to be done for each org you are a part of, if you want to be able to switch between orgs without having to log in each time.