Actually you can add SQLSales1 to the group, but you have to use a different way of addressing it.

So for example if you know that SQLSales1 is listening on port 2001, you can add it as SQLSales1,2001. Change the second field in the New Registered Server dialog back to SQLSales1 and it will be still be displayed under the group using that name.

EDIT: Ha - I see shawnjohnson just beat me to it!