Agency365 allows account holders to have multiple users, each with different tasks and permissions. This allows marketing or advertising agencies among many other businesses to limit the access individual employees have on a large project while still providing them with all the tools they need.
No, all the features are available in SimplyCast 360, however, there is a good deal more training for resellers who are taking this model on. Because of that, the package is only available to Diamond-level resellers.
It is possible in theory, but not explored since at the time of writing, Agency365 wasn't available to all resellers. Once the product is rolled out for all resellers, this would be a discussion to have with the representative you're speaking with.