Opening Up a Service Through an API

I wanted to get advice on opening up a Rails application to allow an
external service to make REST calls to my service. Currently, REST calls
are only made securely through the out of the box Rails security.
However,
I would like to open all of those to external users via an API.

For example, right now people can create blog posts once creating an
account and logging in. However, I would to be able to allow a third
party
to be able to create users and then allow blog posts for users by making
API calls.

I am hoping to get an idea of what are the possibilities and what will
keep
it the secure as possible.

Thanks

Here is some advice for older version of Rails

Any Rails 3 advice?