That requires an aggregate HAVING clause:
$sql = 'SELECT Pos, FName, LName, ROUND((SUM(H)/SUM(AB)),3) AS AVG '
. ' FROM retired_batters '
. ' WHERE Team = \'' . $team_id . '\' '
. ' GROUP BY Pos, FName, LName'
. ' HAVING SUM(AB) > 100'
. ' ORDER BY AVG DESC '
. ' LIMIT 10';
If you use any aggregation (ie: group by on a property and looking for something like a COUNT, SUM, etc), then you always compare it using the HAVING clause. The structure is always: SELECT fields FROM location WHERE where GROUP BY groupby HAVING having ORDER BY order, so you stick having between the group by and order by.