AWS Identity and Access Management is very powerful and very flexible. My colleague Elliot Yamaguchi has written a blog post that shows you how to use IAM to create a policy which implements folder-level permissions within an Amazon S3 bucket. By using this policy, you can allow hundreds of users to safely share a single bucket, restricting each one to a particular folder within the bucket.
The post contains a complete explanation of the policy. You can use it as-is or you can customize it as needed.