i know nothing about the data structure of Invision but i'm going to assume that all the data is stored in a database. you would need to add a conditional statement when a new post is added to check the number a posts and if it equals X number then send email. here's a VERY simplified example:
$result = mysql_query("INSERT INTO posts SET
user_id = '$user_id',
datetime = '$datetime',
subject = '$subject',
message = '$message'");
$result = mysql_query("SELECT COUNT(*) FROM posts WHERE user_id = '$user_id'");
if (mysql_result($result, 0) == 100)
{
mail($email, 'Congratulations!', 'You have made it to the exclusive 100 posters club');
}