Group next class by field CompanyId and add all CampaignId in a list 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; } }

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; }
    }

var query = from e in reportWeeklyDTOs
            group e by e.CompanyId into companyGroup
            select new
            {
                CompanyId = companyGroup.Key,
                CampaignIds = companyGroup.Select(x => x.CampaignId).ToList()
            };