Amazon Identity and Access management (IAM) supports creating sub-users in main Amazon account. Each sub-user has his own access key and secret keys. And admin can define which bucket he can access. The sub-user can access S3 bucket without the knowledge of the main S3 account log credentials.
Gladinet provides a tool, for S3 admin to create sub-users and assign them to different buckets.
You can download the S3 Manager here.
Here is the user guide.
Once the sub-user has the access and secret keys and the correct permission to access the bucket, he can mount the bucket in Gladinet and access it from Gladinet drive.