Credentials popup issue while adding a user to SharePoint group with REST API

Question

I am using SharePoint 2013 On-premise,

We have two SharePoint groups(Group-A, Group-B) in a SharePoint site. Group-A is owner group of Group-B, Generally Group-A users can add,delete users from Group-B.

when Group-A user trying to add a user to Group-B with following REST API code in button click, immediately a credentials popup is coming. when i checked the group B, user was added successfully.

Below is my code on button click. I am using Content editor web part.

function (webAbsoluteUrl, groupId, userLoginName, success, failure) {
        var requestUri = webAbsoluteUrl + "/_api/web/sitegroups(" + groupId + ")/users"
        var userDataItem = {
            __metadata: {
                type: 'SP.User'
            },
            LoginName: userLoginName
        };
        $.ajax({
            url: requestUri,
            type: "POST",
            async: false,
            data: JSON.stringify(userDataItem),
            headers: {
                "accept": "application/json;odata=verbose",
                "content-type": "application/json;odata=verbose",
                "X-RequestDigest": $("#__REQUESTDIGEST").val()
            },
            success: function (data) {
                success(data);
            },
            error: function (data) {
                failure(data);
            }
        });
    }

Here my issue is::

• If Site-collection administrator login into the page, can able to add users to Group-B without any issues(no pop Up).
• Group-A user login into the page, can able add himself to Group-B without any issues(no pop Up).
• But Group-A user login into the page and trying to add other users to Group-B , User was added but immediately showing "SP Credentials pop-up".

I got following error in console::

"{"error":{"code":"-2147024891, System.UnauthorizedAccessException","message":{"lang":"en-US","value":"Access denied. You do not have permission to perform this action or access this resource."}}}"

Answer 1

For adding user to group, instead of REST API I used JSOM, now I didn't get credentials pop up. :)

Adding users to a group with JSOM using following code::

addUserToSharePointGroup: function (loginName, success, failure) {
  var _this = this;
  var clientContext = new SP.ClientContext.get_current();
  var siteGroups = clientContext.get_web().get_siteGroups();
  var web = clientContext.get_web();
  spGroup = siteGroups.getByName(_this.group_AuditProjectMangers);
  user = web.ensureUser(loginName);
  var userCollection = spGroup.get_users();
  userCollection.addUser(user);
  clientContext.load(user);
  clientContext.load(spGroup);
  clientContext.executeQueryAsync(success, failure);
 }

Hope this one might help to others... :)

Ornery Walrus thanks for response....