Once you create some email groups, click the "Project Setup" link under the project name in the header, then click the email groups tab.  They will be displayed in that notification's window after edit / save functions.  We should probably add a link to create your first email group if none exist to make it a bit more clear.

I've added this to our roadmap for discussion. There are some things to consider around security in that users have different rights to different projects so it would be possible that a user could exist in a group and that the group could be added to a project that user does not have rights to.