Yes, it is possible to create an array using the "Cortex user" data source in a User Input block, as long as you enable the "Allow multiple" option while configuring the block:
When this option is enabled, the User Input block will output an array of selected users (as strings), which can then be used in subsequent workflow steps.
If a Scaffolder step appears later in the Workflow and you do not have the option to select multiple users there, this can happen because of how the variable override is configured. If you want a field to be editable in a Scaffolder block, make sure to enable the "Is override editable" toggle while configuring your Scaffolder fields.
Learn more about User Input and Scaffolder blocks in the Workflow blocks documentation.