Per rule 1.3.15: Valid AMA membership cards must be presented at the flying field prior to flying any team’s aircraft.
All pilots must be AMA members, with one exception. Members of the Model Aircraft Association of Canada do not have to join the AMA, as the AMA has a reciprocal arrangement with the MAAC.
One reason for this requirement is the liability insurance provided by the AMA.
You can become an Affiliate member of the AMA, if you are not a USA resident. However, you must apply using a mailed-in form, well in advance of your need for membership. You cannot obtain an Affiliate membership on line. This link the the mail in form mentions Affiliate membership:
If you want a full AMA membership and you are not a US resident, you must also apply using a mailed-in as stated on the online registration web site.
US residents can join the AMA on the main web site as a full member. They do take credit cards and they send an email confirmation of membership right away.