"Yammer works better when more people in a company take part in communicating and sharing. For this reason, Yammer encourages people to send invitations to their coworkers who are not yet on Yammer. You will only receive invitations to Yammer if one or more of your coworkers provided your email address.
From an old thread, for invites that "occurs when a user invites the group to join yammer, it automatically sends invite notification" and that "this is by design. You cannot opt out of receiving email invites from Yammer that is user initiated":