IAM Group is A collection of users who all need the same type of access to a particular set of resources or compartment.
IAM DYNAMIC GROUP is A special type of group that contains resources (such as compute instances) that match rules that you define (thus the membership can change dynamically as matching resources are created or deleted). These instances act as "principal" actors and can make API calls to services according to policies that you write for the dynamic group.
Reference:
https://docs.cloud.oracle.com/en-us/iaas/Content/Identity/Concepts/overview.htm GROUP:
A collection of users who all need the same type of access to a particular set of resources or compartment.