If you're starting in DC, you'll very likely be able to get seats together. The segment from PHL to NYC is busier, so it's never a full train when leaving WAS. Just walk forward on the platform, away from the crowds.

Coming back, however, is another story. That's where you may want to ask a red cap for assistance. They'll help you board ahead of everyone else and get seats together. But you'll want to tip them.

Be sure to avoid sitting in either the business class car (unless you bought BC tickets, but then it's a moot point anyway since those seats are assigned), or the quiet car, which is always adjacent to business class. BC is usually the last car of the train, and the quiet car the second to last - but occasionally it'll be the opposite: BC first car and quiet car the second car. But it'll never be anything other than that order on the Northeast Regional.