How to create iam identity center group connected to an external AD?

You can only use one Active Directory domain with Identity Center, if you want to use more than one, the recommendation is to use AWS Managed Microsoft AD. You can read more about it here:

• https://docs.aws.amazon.com/SetUp/latest/UserGuide/setup-IdCPrereqs.html#:~:text=You%20can%20connect%20only%20one,use%20AWS%20Managed%20Microsoft%20AD.

If you don't want to do this, you would have to add the users to your current AD, create a group and then attach that group to the application as you would normally.