Users

{
    "fields": [{
        "group": "Default",
        "name": "Users",
        "handle": "users", // This line is optional
        "instructions": "This is not a required field.",
        "type": "Users",
        // typesettings & everything inside is optional
        "typesettings": {
            "sources": ["admins"], // Use the handle of the UserGroup or leave blank for all. To select Admins use the string "admins".
            "selectionLabel": "Add a user"
        }
    }]
}