Try this:
var result = from a in TableA.AsEnumerable()
join b in TableB.AsEnumerable() on a.Field<string>("Userid") equals
a.Field<string>("Userid")
select new
{
TempId = a.Field<string>("TempId"),
Email = a.Field<string>("Email"),
PhoneNo = a.Field<string>("PhoneNo"),
Userid= a.Field<string>("Userid"),
Name = b.Field<string>("Name"),
};
--Amit