{
    "match":
    [
         [domain.id", "=", "domain id 1"],
         [id", "=", "user id 1"] // or ["loginName", "=", "john"] etc, any searchable user attributes can be used as the match condition
    ],
    "return":["id", "loginName", "email"]
}

{
    "error":0,
    "result":
    {
        "id":"user id 1", 
        "loginName":"John Smith",
        "email": "john.smith@acme.com"
    }
}