Rule to detect checkbox value when checkboxes are in a group

I'm trying to create a Rule with the Rules module.

I have an Add User form with a checkbox group (4 checkboxes) that was added to the form by the Organic Groups module. Each checkbox is to join a different group, e.g. "Join OG 1", "Join OG 2", etc.

When creating the Rule, how do I:

1. Put in the appropriate reference to the checkbox array in the "Form element ID:" field?
2. Put the appropriate test value in the "Value(s):" field?

I was able to get this working with one checkbox (element ID "account[status]") by testing for value "1".

I've tried all sorts of variations on the OG checkbox group, such as

og_register
og_register[8]
edit-og-register-8

with both single and multiple entries in the "Values(s)" field, nothing seems to work.