You are using the wrong tool. In Assembler you define the cloud template and its inputs. Think of this as an API definition for your service.
In Service Broker you create Custom Forms, which provides the GUI for your service, this is the presentation layer.
So you need to define your input as Array of Strings in Assembler, then go to Service Broker and create a Custom Form for your published content (cloud template). For an Array of Strings input you have 3 options:

Array Input is the default, with a very basic input list where the user can type:

Multi Select: here you can define a predefined list of options the user can choose from. This list can be static or external source (action based).

And Dual List, with a predefined list of options (static/external):

Having an AD query is not possible as the Cloud Template inputs are limited to primitive types (string, number, boolean and object (properties)). The drawback of the above solutions is that you have to list all the users to select from, and an AD can have too much users to display.
XaaS forms (backed by vRO workflows) can have custom resource types as inputs (AD user, for example) where you could use Value Picker, but this is not available for Cloud Templates. What you can do is create a Resource Action of VMs, as it is similar to XaaS forms and there you can add users in the way you described.