It does not seem possible to set or more default users in a custom field of type "users" for usage within Maniphest.
For example, whenever attempting to define a set of user names.

It is only visually possible to attempt setting one or multiple default user(s) by setting an array with the "PHID-USER-*" user values. See the example in the configuration below.

• FYI, this "PHID-USER-*" value is extracted from the the value of the hidden field of the desired user, as rendered in a editable "users" field.

"cust:reviewer": {
  "name": "Reviewer",
  "type": "users",
  "search": true,
  "required": true
  "default": [
    "PHID-USER-wx672b7gtrpq6nflwbis"
  ]
 }

With visually possible, I mean that the configured user will neatly show up in the field as such:

However, the user will not actually be set upon saving the task. Additionally, no error is given when saving the task.

In the configuration example, this custom field is a required field. Maniphest will behave as if this configuration workaround is accepted when saving the task. Upon viewing the task, no user is set in the custom field and editing the task will then result into an accurate required field warning.