Is there any way to prepay for membership? Gift card?
You can only purchase gift cards for apparel, which is clearly not a use that would meet the terms and conditions. Though I'm sure somebody will try it at some point.
You can no longer prepay membership. Ages ago, their referral bonus was a free month's subscription, and until about 2018, you were able to finance the bike together with three years of membership. That's how anybody that has prepaid membership got it.
I believe the only way to share the offer is to give your card to somebody to set as their payment method for monthly payments, and then trust that they cut it off after the right amount of payments post.