There should be an Add User Button that appears in your Customer Portal -> Instance page. Maybe you currently are unable to add colleagues/new users because you are on a trial period?
According to Splunk FAQ's:
Question: If I am using my Splunk.com user login to access my instance, how do I invite a collaborator?
Answer: To invite a user, go to the Instance Page in the Customer Portal and click the "Add User" button.
According to Customer Portal -> Instances Home:
You can invite users to join your instance with a custom role. Create a custom role in the Splunk Cloud product, add a role with the same name in the customer portal page, then choose that role when inviting new users. For more information see the Splunk Cloud documentation.
Not at all - thanks for your response. I can't find any information anywhere saying that you can't invite users on a trial.
As you said - I found that specific link on the FAQs and followed it. Searched the page for "Add" - definitely nothing there (and checked the drop downs)! 😞 Must be an unspoken limitation of the trial (which doesn't sounds right, but there you go)