Would you prefer a walkthrough Bas - a Skype or Hangout? Two things I wanted to check too - (1) I assume your client is a company that already uses Yammer internally? And (2) I also assume your client has a the full paid-for version?
- you can easily add users (but I believe external users cannot add themselves). User management is done through an email address and you can easily add/delete people.
- you can use the API in a number of interesting ways (should you wish) and there are a number of third party app plugins
- There are standard features like keyword alerts
- Users can create their own groups based on interest, but the admin has control over them overall.
- Users can choose from a good selection of notification settings.
- And, of course, a great thing is there's a ready-made good functionality mobile app for Android, iOS (different for iPad and iPhone) and Windows phone.