You can add new users, remove existing users, and update the scopes of existing users.
The owner is a special type of user. Exactly one user can have the owner scope. And every application has to have an owner. You can edit the owner's scopes, but only the owner can transfer ownership to a different user (the desired new owner must already be part of the application). This allows you to define the scopes you want the current owner to have after ownership has been transferred. Furthermore, you cannot remove the owner from the application: you must first transfer ownership to a different user.
Only owners and users with the admin:full scope can manage an application's users.