IAMS user can't access Cloud9 IDE

Generally it's against best practices to use root credentials to create and share environments because read/write members can use the environment owner's AWS access credentials. See here for more information. Specifically this section:

For EC2 environments, read/write members can use the environment owner's AWS access credentials, instead of their own credentials, to make calls from the environment to AWS services. To prevent this, the environment owner can disable AWS managed temporary credentials for the environment. However, this also prevents the environment owner from making calls. For more information, see AWS Managed Temporary Credentials.

As for your question, what's the IAM policy look like for your IAM user(s)? Are you using an AWS Managed Policy like AWSCloud9User or are you using a custom policy? If using a custom policy, ensure it includes the permissions similar to those listed out in the AWSCloud9User as those will be required to see and access environments, whether they are shared or not.