Unfortunately I gave up diigo recently. Too buggy, in short. The sum of "tagged" and "untagged" bookmarks doesn't equal the number of "all" bookmarks. Same with "public" and "private". "Annotated" bookmarks - 27, and yet counting the list manually nets 16. And many more. I finally gave up and open a new account to start afresh, no good. Try again, still no good.
I asked for help on their forum, but didn't get any. (To be fair, their support forum is better staffed than many, with two active regulars. But still questions and requests could go unanswered.) I finally called it quit. Wasted too much time on it already. It's a shame, for I like the interface and the toolbar in general.
After some digging, however, I've found I could duplicate all the features I want locally and more, with a set of Firefox extensions. Some aren't perfect. Wired-Marker is a little more cumbersome than diigo's highlight tool, and its data is stored locally, which is a plus (no worries if diigo disappear) and a minus (harder to sync) at the same time. And Tweez (for tag autocompletion) has an awkward interface.
But in general it works, and I get to tap into FF3's much improved bookmark system, which, with help from extensions, beats diigo or any other social bookmarking service in many areas. (E.g., try deleting a tag from selected bookmarks, or opening several bookmarks in tabs at the same time. The latter has been requested several times since months ago, and they keep saying it's coming.) I also get the peace of mind of one less company tracking my web usage.
None of this has anything to do with diigo's group feature. Just feel responsible to share since I recommended them earlier.