Ask HN: Has anyone built an API or Service that handles invitation workflows

In the last few startups we needed a way for our users to invite other users. The details differ, but in rough lines the process is similar. You can see this in action when you add people to slack, your github, bitbucket, trello, or whatever "social" or "group based" software you use.

  - User Anne, invites Bob by providing an email-address or phone number.
  - Service Foo then sends out a message including a secret token (in the form of an url or a number)
  - Bob follows the url, or otherwise provides the token to service Foo.
  - Bob is now confirmed (actually, it is confirmed he has access to the communication channel provided by Anne)

I was wonder if an API for this exists, as a SAAS solution, or even open source microservice kindof thing. Anything really.

The reason is not that I want to use (allthough I do) but because I'm looking into building it.

So, the next question would be: would anyone be interested in using such a service? Obviously the details matter a lot, like the exact setup, workflows, storage of user data, and features such as the invitation channels (mail, SMS, telegram, etc), but as a general idea: would you consider buying/paying for an external service for your next "invitation" workflow?