can I scale up (add more nodes) my Kubernetes cluster in AWS via AWS Auto Scaling Group only or I must use kops for this purpose as well? I used kops to provision Kubernetes cluster in the first place.
You definetly you can scale up in aws EKS using aws auto scaling group.
cluster auto-scaler will automatically add the node in pool when there is insufficient memory for pod or there fails to assign new pod on node.
for more help you can refer this document : https://medium.com/@alejandro.millan.frias/cluster-autoscaler-in-amazon-eks-d9f787176519