https://docs.opsmanager.mongodb.com/current/reference/api/user-create-first/

This page states the following:

Create the first Ops Manager user. This endpoint is available only when the Ops Manager instance has no users. This is the only API call you can make without first having an API key.

It's true that this resource is the only way to create the first Ops Manager user, and that user will be given the GLOBAL_OWNER role. However, it can also be used to create additional users. Any users created after the first one will not be given the GLOBAL_OWNER role.

There are two exceptions:

1. You cannot use this resource when Ops Manager is configured to use LDAP for auth.
2. You can only use this resource to create the first user when Ops Manager is running in invitation-only mode.