When you attach instances, Auto Scaling increases the desired capacity of the group by the number of instances being attached. If the number of instances being attached plus the desired capacity of the group exceeds the maximum size of the group, the operation fails.
If there is a Classic load balancer attached to your Auto Scaling group, the instances are also registered with the load balancer. If there are target groups attached to your Auto Scaling group, the instances are also registered with the target groups.
For more information, see Attach EC2 Instances to Your Auto Scaling Group in the Auto Scaling User Guide.