How do I use `group by` in X++?

Use group by with aggregate functions when you want one result per grouped value.

SalesLine salesLine;

while select ItemId, sum(LineAmount) from salesLine
    group by ItemId
{
    info(strFmt('%1 - %2', salesLine.ItemId, salesLine.LineAmount));
}

The aggregate result is returned through the field buffer you aggregated. Any non-aggregate field you select needs to be part of the grouping.