This formula uses the value in cell E5 for a lookup value, the named range "key" (H5:I9) for the lookup table, 2 to indicate "2nd column", and 0 as the last argument indicate exact match. You can also use FALSE instead of zero if you like.
VLOOKUP simply looks up the value and returns the group name from the 2nd column in the table. Both columns in the table can contain any values that you need, and the table doesn't need to be sorted since VLOOKUP is using exact match.
As always, if VLOOKUP can't find a match, it will return #N/A.