public class EmailCompanyContact
    {
        public int contactID { get; set; }
        public int CompanyID { get; set; }
        public string contactName { get; set; }
        public string contactEmail { get; set; }
    }

List<EmailCompanyContact> lEmailCompanyContact = new List<EmailCompanyContact>();

var EmailCompanyContact = from p in lEmailCompanyContact where p.CompanyID == Convert.ToInt32(gvCompany.SelectedDataKey["CompanyID"]) group p by p.contactID into g select new { g.Key, g};

gvContacts.DataSource = EmailCompanyContact;
gvContacts.DataBind();

var q = lEmailCompanyContact.GroupBy(pe => pe.contactID, pe => pe).AsQueryable();

gvContacts.DataSource = q;
gvContacts.DataBind();

.....
<Columns>
                                
                                <asp:BoundField DataField="contactName" HeaderText="Contact Name" SortExpression="contactName">
                                    <HeaderStyle HorizontalAlign="Left" />
                                </asp:BoundField>
                                <asp:BoundField DataField="contactEmail" HeaderText="Contact Email" SortExpression="contactEmail">
                                    <HeaderStyle HorizontalAlign="Left" />
                                </asp:BoundField>
                            </Columns>

var listCompanyContact = from p in lEmailCompanyContact
                                         group p by p.contactID into g
                                         select new { contactID = g.Key, Item = g.ToList() };

                var joinList = from contact in listCompanyContact
                               join p in lEmailCompanyContact on contact.contactID equals p.contactID into contact2
                               from item in contact2.DefaultIfEmpty(new EmailCompanyContact())
                               select new { contactID = contact.contactID, CompanyID = item.CompanyID, contactName = item.contactName, contactEmail = item.contactEmail };