from m in Membership.GetAllUsers().Cast<MembershipUser>()
                                  join u in ClientToUser.All() on m.ProviderUserKey equals u.userId 
                                  join c in Client.All() on u.clientId equals c.clientId 

                                  select new UserClientModel { Client = c.clientName, Email = m.Email, LastActivityDate = m.LastActivityDate, UserName = m.UserName };

 from m in Membership.GetAllUsers().Cast<MembershipUser>()
                                  join u in ClientToUser.All().DefaultIfEmpty() on m.ProviderUserKey equals u.userId 
                                  join c in Client.All() on u.clientId equals c.clientId 

                                  select new UserClientModel { Client = c.clientName, Email = m.Email, LastActivityDate = m.LastActivityDate, UserName = m.UserName };