Another possibility (although perhaps tricky in the real world). Work with the local council (who already have all the constituents details) so that everyone automatically becomes a library member and a card and details are included in their next rates / tax / whatever postal mail. Could be sold on literacy grounds for the council and from the client side details of the free online acess to streaming / magazines / books / Encylopaedia Britannica etc (depending on what the library subscribes to) would be provided to get engagement