Group next class by field CompanyId and add all CampaignId in a list Linq lambda public class ReportWeeklyDTO { public string CampaignId { get; set; } public string CampaignName { get; set; } public int NumberMessages { get; set; } public string User { get; set; } public int SentMessages { get; set; } public int ErrorMessages { get; set; } public string ErrorDescription { get; set; } public string CompanyId { get; set; } }

return reports.GroupBy(x => x.CompanyId).Select(x => new ReportWeeklyDTO
                                {
                                    CampaignId = string.Join(",", x.Select(y => y.CampaignId).ToList()),
                                    CampaignName = string.Join(",", x.Select(y => y.CampaignName).ToList()),
                                    NumberMessages = x.Sum(y => y.NumberMessages),
                                    User = x.First().User,
                                    SentMessages = x.Sum(y => y.SentMessages),
                                    ErrorMessages = x.Sum(y => y.ErrorMessages),
                                    CompanyId = x.First().CompanyId,
                                    ErrorDescription = x.Sum(y => y.ErrorMessages) > 0 ? string.Join(",", x.Where(y => !string.IsNullOrEmpty(y.ErrorDescription)).Select(y => y.ErrorDescription).ToList()) : null
                                }).ToList();