Sell memberships offline

I´m developing a coupon site with membership plugin. I have local companies offering vouchers on my site and visitors need to become members to take advantage of the offers.

I would like to expand this idea and sell memberships offline. The main purpose would be to get sports teams and school classes to sell memberships in order to earn some money for a trip or something similar.

How could this be done? I figured invitation codes printed on cards. Does that work with membership?

Another way would be to use some sort of affiliate plugin, any ideas on how to implement that with membership?