I too have different experiences. Sometimes I'm given a seat number, and other times I'm simply told "upstairs and to your right". If you are given the latter instruction, simply look for seats that are both empty and don't have a seat check (little paper stub) hanging above from the luggage bin.
If you want to sit together, and the attendant gives you seats that are apart from one another, go ahead and take the assigned seats. After the attendant and the conductor have everyone settled in, simply inform the attendant that you would like to have seats together as soon as they're available. In many cases, the attendant will go ahead and try to re-assign seats. In other cases, the attendant may have to wait until the next major stop when the train has large turnover. In some rare cases, the train may be so filled that re-assignments are not possible. But don't worry, you'll have plenty of opportunities to visit with the rest of your party in the dining car and in the lounge car.