How do I get other users from my organizations associated to our support contract?
if you have an Enterprise license, you have a set number of 'authorized contacts' that you can associate with your account as part of your Support contract. one or more of these authorized contacts can be designated as a 'Portal Admin'. if you are logged in as a "Portal admin" to the Support Portal, you can add authorized contacts up to your limit.
an important point: those users must already exist as part of your company's account. if they don't, as lukejadamec points out, you should contact Splunk Support.
You would contact your Splunk Account rep. If you can't remember who that is, then just call Splunk sales.