How can the user configure this with the security group?
There is no need for a security group modification as all the instances can communicate with each other inside the same subnet
Configure the subnet as the source in the security group and allow traffic on all the protocols and ports
Configure the security group itself as the source and allow traffic on all the protocols and ports
The user has to use VPC peering to configure this
Explanations:
Instances in the same subnet require a security group rule to communicate; subnet proximity alone does not grant access.
Specifying the subnet as the source allows traffic from any instance in the subnet, not limited to the specific security group.
Using the security group itself as the source allows communication only among instances associated with the same security group.
VPC peering is not needed for communication within the same VPC or subnet; it is used to connect different VPCs.
I sort that the answer is:
Configure the security group itself as the source and allow traffic on all the protocols and ports