I don't know much about PQ etc but have you tried adding a conditional column?
let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Manager", type text}, {"Headcount", Int64.Type}}),
#"Added Conditional Column" = Table.AddColumn(#"Changed Type", "Band", each if [Headcount] <= 5 then "1-5 Staff" else if [Headcount] <= 10 then "6-10 Staff" else if [Headcount] <= 15 then "11-15 Staff" else "More than 15 Staff")
in
#"Added Conditional Column"
the Group element on the G&T is really about thinning the dataset relative to existing values.
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