Error with parameters not existing?
declare @userid as integer;
insert into users (username,emailaddress,passwordhash,salt) values
 (@username,@email,@password,@salt);

set @userid=scope_identity();
insert into groups (name,userid) values ('somegroup',@userid);
select @userid;

cmd.Parameters.Add(new SqlParameter("@email",user.EmailAddress));
cmd.Parameters.Add(new SqlParameter("@username",user.Username));
cmd.Parameters.Add(new SqlParameter("@salt",user.Salt));
cmd.Parameters.Add(new SqlParameter("@password", user.PasswordHash));

cmd.Parameters.Add(new SqlParameter("@email",user.EmailAddress ?? ""));
cmd.Parameters.Add(new SqlParameter("@username",user.Username));
cmd.Parameters.Add(new SqlParameter("@salt",user.Salt ?? ""));
cmd.Parameters.Add(new SqlParameter("@password", user.PasswordHash ?? ""));

cmd.Parameters.Add(new SqlParameter("@email",user.EmailAddress));

using (TransactionScope scope = new TransactionScope())
{
      //Your code should be here in this case, as you are not using SP.
      //So this will help you in case of multiple users
)