I used Maitre. Super simple to install as it's a widget that can be embedded on any webpage. People are rewarded when signing up their friends by getting ahead in the queue. Design is adapted from the block you put the code into, however, you can customise most of the stuff from your Maitre dashboard. It also integrates natively with MailChimp and supports webhooks & callbacks, so that you can really customise it they way you want. Manuel @mnlfrgr (Founder of Maitre) is super helpful whenever in need of a hand.