If you add another admin in the cloud control panel, is an account for that user created on all cloud instances that person can access?
In Vultr, there aren't users in the cloud panel at all. There are keys that you can choose to deploy at deploy time for root. Other than that, if we wanted to deploy keys (as an example), we'd do that through our management system (script, Salt, Ansible, etc.). I would not want the cloud platform to be touching my users.
OK, got it.
Does that also mean that only one person can have access to the actual Vultr account as well? I'm guessing it's multi-user.
Yeah, the cloud level is multi-user. But just as you can have multiple people with access to a data closet, and multiple people with access to a Windows instance housed in that closet, you don't want the physical closet to maintain the Windows logins. Same here, your cloud provider is like a data center or data closet with its own level of access unrelated to applications or other workloads running higher up the stack and 99.999% of the time, no association or commonality between them.