You can use Profile manager to create an admin only group profile field. This will be a message per group, if you wish to show the same message on all groups i don't know any plugin right now.

Thanks Jerome!

I actually used the 'free html' widget from the 'widget pack' plugin.

I had to add small modification on it to check if the user is admin and enabled the widget for group_profile only.

It works, not perfect because I didn't want it to be a widget, but works! :)

that works too ;)

Solr plugin shows you all the statistics in the admin area. You probably need to build the index.

I've already hit the "full reindex" button in the admin page... that didn't help

is that what you meant?

Yes. I suppose you didn't set Some up correctly. Check the logs for errors