How to remove user from Sharepoint Group using JavaScript?

The error:

The user does not exist or is not unique

most probably occurs since user could not be found.

The list of methods that could be utilized for removing a user from group:

• SP.UserCollection.remove
• SP.UserCollection.removeById
• SP.UserCollection.removeByLoginName

The following examples demonstrate how to remove User from Group using SP.UserCollection.removeByLoginName

JSOM example

function removeUserFromGroup(userLoginName,groupName,success,error)
{
    var ctx = SP.ClientContext.get_current()
    var web = ctx.get_web();
    var group = web.get_siteGroups().getByName(groupName);
    group.get_users().removeByLoginName(userLoginName); 
    ctx.executeQueryAsync( 
       function(){
           success();
       },
       error);
}

Usage

var userLoginName = 'i:0#.f|membership|[email protected]';
removeUserFromGroup(userLoginName,'Team Site Members',
function(){
    console.log('User has been removed from Group');
},
function(sender,args){
    console.log(args.get_message());
});  

REST example

function removeUserFromGroup(webUrl,userLoginName,groupName)
{
    var url = webUrl + "/_api/web/sitegroups/getbyname('" +  encodeURIComponent(groupName) + "')/users/removebyloginname(@u)?@u='" + encodeURIComponent(userLoginName) + "'";
    return $.ajax({
        url: url,
        method: "POST",
        contentType: "application/json;odata=verbose",
        headers: {
           "Accept": "application/json;odata=verbose",
           "X-RequestDigest": $("#__REQUESTDIGEST").val()
        }
    });
}

Usage

var userLoginName = 'i:0#.f|membership|[email protected]';
var groupName = 'Team Site Members';
removeUserFromGroup(_spPageContextInfo.webAbsoluteUrl,userLoginName,groupName)
.done(function(data)
{
   console.log('User has been removed from Group');
})
.fail(
function(error){
    console.log(JSON.stringify(error));
});

Using REST

You can REST endpoints for removing users from groups

http://msdn.microsoft.com/en-us/library/office/dn531432(v=office.15).aspx#bk_UserCollectionRemoveByLoginName

executor.executeAsync({
  url: "<app web url>/_api/SP.AppContextSite(@target)/web/sitegroups(7)/users
    /removebyloginname(@v)?@v='i%3A0%23.f%7Cmembership%7Cuser%40domain.onmicrosoft.com'
    &@target='<host web url>'",
  method: "POST",
  success: successHandler,
  error: errorHandler
});

Using Server Object Model

web.AllowUnsafeUpdates = true;
SPGroupCollection collGroups = web.SiteGroups;
SPUserCollection usercoll = siteobj.RootWeb.SiteUsers;
int ID = oWeb.Users[UserName].ID;
usercoll.RemoveByID(ID);
web.Update();
web.AllowUnsafeUpdates = false;

Using Client Side Object Model

ClientContext client = new ClientContext(SITEURL);
client.ExecuteQuery();
Web website = client.Web;
client.Load(website, w => w.AllProperties, w => w.SiteGroups, w => w.SiteUserInfoList, w => w.Webs,w => w.Title);
client.ExecuteQuery();
GroupCollection groupCollection = website.SiteGroups;
client.Load(groupCollection, groups => groups.Include(grps => grps.Users, grps => grps.Title));
client.ExecuteQuery();
User user;
foreach (Group group in groupCollection)
{
   if (group.Title.Equals("GroupName"))
   {
        user = group.Users.GetByLoginName(“UserName”);
        group.Users.Remove(user);           
        group.Update();
        website.Update();
        client.ExecuteQuery();
        break;
    }
}

Using JavaScript Object Model

var clientContext = new SP.ClientContext(siteUrl);
var oWeb = clientContext.get_web()
var collGroup = oWeb.get_siteGroups();
var oGroup = collGroup.getById(7); //Change 7 based on the group's id
var oUser = oGroup.get_users();
clientContext.load(oUser);
var user = web.ensureUser("domain\username);
oUser.removeByLoginName(user.get_loginName());   

clientContext.executeQueryAsync(Function.createDelegate(this, onQuerySucceeded), Function.createDelegate(this, onQueryFailed));

function onQuerySucceeded(){
    alert('User Removed');
}

function onQueryFailed() {
    alert('Failed');
}