Hi Don,
One would think that would be a great option for Partners to have....but, nope. It's only available for "free" for clients with a current Microsoft Enterprise Agreement.
If you do not have an enterprise agreement (even partners), you need to pay for the Yammer Enterprise edition in order to enable this in CRM. And having an E1-4 Office 365 account does not count towards having an "Enterprise Agreement" - different animal.
I hope this will change in the near future. Rumor's are a big push for Yammer with Partners is coming from Microsoft.
Mike