+------+------+----------+
|  P1  |  P2  | SomeData |
+------+------+----------+
| Bob  | Mark | data     |
| NULL | Bob  | data     |
| Mark | Tony | data     |
| Mark | Sam  | data     |
| Mark | Bill | data     |
+------+------+----------+

+------+-------+
|  P   | Counts|
+------+-------+
| Bob  |   2   |
| Mark |   3   |
+------+-------+

SELECT
    (CASE WHEN P1 IS NOT NULL THEN P1 ELSE P2 END) AS P,
    COUNT(CASE WHEN SomeData IS NOT NULL THEN 1 ELSE 0 END) AS Counts
    FROM MyTable
    GROUP BY P

SELECT
CASE WHEN P1 IS NOT NULL THEN P1 ELSE P2 END AS P,
COUNT(SomeData) AS Counts
FROM MyTable
GROUP BY CASE WHEN P1 IS NOT NULL THEN P1 ELSE P2 END