Use This in REST Client: HTTPGet Method with below request: http://localhost:8080/nuxeo/site/api/v1/user/

ex: http://localhost:8080/nuxeo/site/api/v1/user/Administrator Result will be:

{
    "entity-type": "user",
    "id": "Administrator",
    "properties":
    {
        "tenantId": null,
        "lastName": "",
        "username": "Administrator",
        "email": "Administrator@example.com",
        "company": "",
        "firstName": "",
        "password": "",
        "groups":
        [
            "administrators"
        ]
    },
    "extendedGroups":
    [
        {
            "name": "administrators",
            "label": "Administrators group",
            "url": "group/administrators"
        }
    ],
    "isAdministrator": true,
    "isAnonymous": false
}