I'm looking for someone who offers "invite a friend" as-a-service. Having looked into the options for my site, which is drupal based, it seems like there are no great options and this isn't really something I want to have to keep up with code-wise. I just want people to be able to invite their friends to join the site and would be willing to pay a monthly fee or what not for this service. Anyone know of any good options? Or, alternatively, any software I can install that will do this (I've been looking at OpenInviter but it seems it may not be well maintained currently - it's sourceforge page has no downloads, and the last tweet from openinviter was from 2010). Thanks.