(http://i60.tinypic.com/2vwtzsy.jpg)
Last year, Microsoft announced that they will be offering Yammer Enterprise to all Office 365 Enterprise plans customers. Today they announced that Yammer Enterprise has been added to all Office 365 Midsize Business and Office 365 Education plans. This addition should further increase the value which Office 365 brings to its customers.
As part of this update, all existing Office 365 Midsize Business and Office 365 Education plan customers will receive licenses for Yammer Enterprise. See Yammer Plans & Pricing for additional information. In addition, Midsize Business and Education customers are no longer required to purchase Yammer Enterprise licenses for external users within their external networks. This also applies to Enterprise customers, as we announced last November. This simple licensing change significantly reduces the friction in cross-organization collaboration and will enable your users to work with customers, partners, and parents and students without having to worry about additional costs.
source: Office blogs