Must party members be present for coop achievements?

If all other party members are in town, in another zone, or dead during an event OR boss battle (one that awards a "cooperative achievement"), will the event/boss battle still award the coop achievement for the whole party?

For example, the other party member in a two-person party stays in town during [an event]—does he still get the achievement, and do both get the cooperative achievement even though the other party member wasn't present? Or everyone declines a boss battle while one person finishes it—do all party members get the achievement AND the coop achievement?

Solution 1:

In my experience, both members have to be with each other to receive the achievement when it involves killing something or exploring, etc.

Example: When I was playing Co-op with a friend, he killed his first rare monster which triggered an achievement and I was back in town - clearing my inventory and repairing. I did not get this achievement (I had to earn this at a different time).

Although, if the achievement is quest related - all members of the party complete the quest at the same time which [I would think] should trigger that specific achievement.