Is a credit card mandatory?
 

I've thought about signing up for a JMB account, however, a friend told me that US citizens can only get a JMB account if you also get one of the JMB-related credit cards (which I don't need). I understand that getting the credit card would allow you to accumulate more miles from everyday purchases (like any other program) but I wouldn't think that this is mandatory just to join. Can I join without applying for a credit card?

Your friend is incorrect. You can sign up for JMB through the Americas region website: https://www.ar.jal.co.jp/ar/en/jmb/index01.html The "JAL Mileage Bank Card (JMB Card)" is not a credit card, just a membership.

Beltway is correct. You definitely don't need one to open the JMB account.