Unless you're seeing some special offering that I'm not seeing, there is no bonus for joining. If you travel within 90 days of joining, then you'll get a 500 point bonus, in addition to whatever points are earned for the actual train trip.

Now to your questions.

It is free to join and it costs nothing to maintain your membership, other than at least 1 ride on Amtrak every 3 years.

If you book at least 3 days prior to your trip, then you can use code H431 to save 10% off of the ticket price. Please note, if you have a AAA card, then you can also combine that discount with the code above.

I'm not aware of any referal programs.