It sounds that he was hired AFTER the startup already had 150 employees as well.
The use of the past perfect (Had grown) is there to express that something happened in the past, before somethign else in a nearer past.
In other words, it says "I was hired" which means this would be the closest past situation in the sentence. The past perfect tense ("had grown") puts emphasis on something prior to the closest past situation (the hiring). If the verb "grown" would've been in the perfect tense (has grown), then it would mean that the department would've expanded to 150 because of this newdirector. I hope it helps.