Can you GROUP BY with a CASE WHEN THEN alias name?

You need to use the whole CASE statement in the GROUP BY clause if you don’t wrapped it in a subquery.

SELECT  CASE 
            WHEN DATEDIFF(o.EndDate, o.StartDate) < 30 THEN '<1 Month'
            WHEN DATEDIFF(o.EndDate, o.StartDate) < 90 THEN '1 - 2 Months'
            WHEN DATEDIFF(o.EndDate, o.StartDate) < 210 THEN '3 - 4 Months'
            ELSE '>4 Months' 
        END AS `Length`, 
        COUNT(DISTINCT(person.ID)) AS `COUNT`
FROM    person
        INNER JOIN opportunity AS o
            ON person.EntityID = o.id
        INNER JOIN Organization AS org
            ON o.OrganizationID = Org.ID
WHERE   person.TitleID = 2
        AND o.bID = 1
GROUP   BY  CASE 
                WHEN DATEDIFF(o.EndDate, o.StartDate) < 30 THEN '<1 Month'
                WHEN DATEDIFF(o.EndDate, o.StartDate) < 90 THEN '1 - 2 Months'
                WHEN DATEDIFF(o.EndDate, o.StartDate) < 210 THEN '3 - 4 Months'
                ELSE '>4 Months' 
            END
ORDER   BY Length ASC;

Remove also the single quotes around the column name in the ORDER BY clause.