You can find and edit the valid 'stats users' for your domain by going to Status -> Site Statistics in the panel and clicking on the link under the Stats Users column.
You add sub-domains to the DreamHost system exactly the same way as you would add a full domain, via the panel at Domains -> Manage Domains -> Add New Domain / Sub-Domain. Enter the full sub-domain in the Domain to host: field then click the Fully host this domain now! button. You can optionally change the user that controls the domain before clicking the button.
Once you have done this a new sub-directory will appear in your home directory with the same name as the sub-domain, this is the web accessable directory for this sub-domain. Note, it may take a little while for the sub-domain to propagate and be usable.
PS: Welcome to DreamHost
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