Amfleet is correct that just by having a Guest Rewards account and traveling every so often, they send you bonus coupons. Sometimes they are for a free drink, other times it's for the Club Acela Lounges on a day that you are traveling on Amtrak. Also when you reach the Select status of GR, they send you a bunch of passes in your kit. They also include a couple of free upgrades from BC to FC on the Acela's.
If you traveled on one train FC and are switching to another train in a city with a lounge, you are entitled to use the lounge even if your second train is not in FC. Also if one happens to have a Platinum American Express card, you can use Club Acela when you show your Amtrak ticket. I've also seen groups granted the use of the lounge. That happened once in DC when I was there, a church group of about 10 people was traveling on the Capital. Even though they were in coach, Amtrak allowed them to use the Club Acela lounge in DC.
There was also talk of setting up a club that you could join for about 40 - 50 bucks, that would entitle you to use the lounge. Don't know if that idea ever got off the ground though as I haven't heard anything more about it.