An IAM group is a:
group of EC2 machines that gain the permissions specified in the group.
collection of IAM users.
guide for IAM users.
collection of AWS accounts.
Explanations:
An IAM group is not related to EC2 machines. It is used to manage permissions for a collection of IAM users, not machines.
An IAM group is indeed a collection of IAM users. Groups allow you to assign permissions to users collectively.
An IAM group is not a guide for IAM users. It is a collection of users to manage their permissions.
An IAM group is not a collection of AWS accounts. It is a grouping mechanism for IAM users within an AWS account.
I measure that the answer is:
collection of IAM users.