I mean not a wordpress plugin, but a tool which may be integrated to the solid platform based website on Ruby-on-Rails, substitution or complementing website sign-up form. Could you recommend one please?
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. The design will be 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.