How can all accounts under an organziation see EC2 instances created by other accounts?

If by viewing, you mean seeing an EC2 instance in the AWS management console, that's not possible. At any one time, you can only see the EC2 instances in the account where your IAM user or IAM role resides. If you want to see EC2 instances in other accounts, you have to assume a role in or log in to the target account.

An AWS account is intended as a high administrative boundary. It's intentional that many resources cannot be shared or operations performed across accounts, and for the limited set of operations for which it is possible, it has to be explicitly permitted.