Grouping Values by Ranges Retreived from a Different Table under Two Conditions

Thank you for the file. See if this works for you.

A. As a calculated column

Headcount Range =
VAR _Threshold =
    CALCULATE (
        MIN ( 'Headcount Ranges'[Upper Threshold] ),
        FILTER (
            'Headcount Ranges',
            'Headcount Ranges'[Upper Threshold] >= 'Company List'[Headcount]
        )
    ) // Calculates the minimum upper threshold value in the Headcount table which is greater or equal to the Company list[Headcount]
RETURN
    LOOKUPVALUE (
        'Headcount Ranges'[Range],
        'Headcount Ranges'[Upper Threshold], _Threshold
    )
// Retruns the range value on the Headcount Ranges table where the upper threshold is the same as the calculated _Threshold value 

B. As a measure

Headcount Range (measure) =
VAR _Threshold =
    CALCULATE (
        MIN ( 'Headcount Ranges'[Upper Threshold] ),
        FILTER (
            'Headcount Ranges',
            'Headcount Ranges'[Upper Threshold] >= SUM ( 'Company List'[Headcount] )
        )
    )
RETURN
    LOOKUPVALUE (
        'Headcount Ranges'[Range],
        'Headcount Ranges'[Upper Threshold], _Threshold
    )

Sample PBIX file attached