How to set a rule that assigns a user to a specific Organic Group after the checkout process I´m using rules, organic groups and drupal commerce modules.

I´ve set a rule that creates a new user after the anonymous completes the checkout process.

I want the rule to assign the user to a specific group (OG). I can´t find the way to do it.
I´ve added a screenshot of what are the available options,a nd it seems that there aren´t enough available selectors to do that.

Is there a way to add the user using php? How?
I´ve added the print screen after setting Action: Add entity to group.
For reference, this type of use case (selling access to an organic group) would be best handled now using the Commerce License framework, that has been designed specifically for this type of use cases.

We don't have integration with Organic Groups yet, but it seems straightforward to do in the framework.