Here’s how you can add an existing instance to a new Auto Scaling group:

Open EC2 console

Select your instance under Instances

Choose Actions -> Instance Settings -> Attach to Auto Scaling Group

Select a new Auto Scaling group

Attach this group to the Instance

Edit the Instance if needed

Once done, you can successfully add the instance to a new Auto Scaling group

https://docs.aws.amazon.com/autoscaling/ec2/userguide/attach-instance-asg.html