Service to Allow Users to Invite Friends to my Site

Similar to LinkedIn, Twitter, etc

[physics]

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.

[Hoople]

Correct me if I'm wrong but isn't that what the 'email a friend' part of most social sharing widgets for websites was made for?

[physics]

I'm thinking more along the lines of how sites like LinkedIn allow you to invite many people at once.

[physics]

BTW, since posting this I've been looking at a service called LoginRadius which has social login and sharing options. For some reason finding the right combination of keywords to find companies like this was very difficult :p Anyway, this is the sort of thing I'm looking for ... anyone have experience with this or similar companies?