Learn Russian: Все друзья привстали, когда вошёл директор. - All the friends half-rose when the director entered.

The prefix при- here is used as an attenuative (смягчительный). It indicates that an action was done only slightly or incompletely. Instead of fully standing up (встали), the friends only half-rose or lifted themselves slightly out of their seats.

Is привстали perfective or imperfective?

It is perfective, focusing on the completed result of the action in that specific moment. The imperfective pair for this verb is привставать.

Why is the word order вошёл директор instead of директор вошёл?

Russian naturally places the newest or most important information at the end of a clause. By putting директор last, the sentence emphasizes exactly who it was that entered.

As a refresher, why use вошёл here instead of зашёл?

вошёл emphasizes the physical act of crossing the threshold into the room. зашёл is often used for dropping by for a short visit. Given the formal context of a director arriving, вошёл is the most neutral and precise choice for entered.