My fellow Office 365 MVP and friend (and someone with a much more level head than I) has done a great blog post comparing Yammer to Exchange distribution lists and walking you through how to make changes or perform functions to make Yammer work the way you are comfortable with.
As a person who loves to email, and loves distribution lists – I have done exactly these steps within most of the networks on Yammer I belong to. I strongly suggest if you want to stay working with distribution lists the way you do now but are being moved to Yammer – follow this article.
http://blog.atwork.at/post/2014/12/07/Comparing-Yammer-to-Exchange-Distribution-lists.aspx