How to setup AWS services visible to others in my organization

I am trying to setup AWS services for my organization. I have a test User Pool in AWS Cognito that I am trying to share admin rights with my teammate. However he cannot see my created User Pool when he logs in with his AWS account.

We are tiny, with two developers - neither of which have ever used AWS. Following the documentation, I have created a professional account, my teammate has created a professional account, I have created an organization (where I am the root/account manager) and have invited my teammate into the organization (he has accepted). I have created an Organizational Unit and a policy explicitly allowing access to the Cognito Service.

What could I be doing wrong? I don't see any options for personal vs organizational server usage - we are both flummoxed at googling for hours a tutorial that would help us with this most basic operation.

You are making it too complicated. Instead of joining AWS accounts into Organisation and trying to do cross-account access - which is possible but somewhat complicated - I suggest you simply create 2 IAM User accounts. One for you and one for your teammate, give yourselves Administrator privileges and don't forget to set up MFA (Multi-Factor Authentication) to protect your accounts. Don't use the root account that's identified with the email address for the day to day operations. Always use the IAM users.

Hope that helps :)