Borderline feature request, since it's not a crasher or a work-blocker, but the behavior is maddeningly and confusingly inconsistent, so I thought I'd give it a go. Here's a quick walkthrough I tested on three different builds; the weird behavior is evident:

Attached is a .BLEND with a simple compo network with a group with "Backdrop" enabled. Please open and F12 to fill the buffer from the factory default scene. In the compo network, Switch the "Viewer" node input noodle from the node "Hue Saturation Value" to the NodeGroup to see the desired behavior of updating the Viewer and the Backdrop. Now select and tab open the NodeGroup. Switch the internal noodle attached to "Color_out" from the bottom "Add" node to the top "Add" node. The "Compositing" progress bar shows, but ends instantly without updating the external viewer or the Backdrop. (There's also no update to the Compositor if the NodeGroup is connected to the Composite out node.) Lacking this visual refresh, the network is now "dirty," with the Viewer node and the backdrop not showing the result of the network shown, which is confusing for the artist. Tabbing in and out of the NodeGroup will again flash the "Compositing" progress bar, but no update is made.

Now, add a new Viewer node inside the NodeGroup. This operation exhibits the expected behavior of updating the Backdrop, and the Viewer inside the NodeGroup, but not the viewer outside the NodeGroup. Switch the input noodle of the new Viewer to the buttom Add node, and again note that no updates are drawn; again the Viewer and Backdrop are showing "the wrong thing." (And now the Viewer outside the Node Group is two changes behind!) Tabbing into and out of the NodeGroup doesn't draw and update. Now, in the 3D View, tab into Edit Mode, then tab back into Object Mode. In the compositor, tab out of the NodeGroup. Now the external Viewer refreshes! But now open the NodeGroup again ... and that pesky internal Viewer is still a little confused.

P.S. It's somewhat maddening that clicks inside a NodeGroup can "fall through" to affect controls sitting "behind" the expanded nodegroup -- this is inconsistent with the otherwise extant UI principle that while a nodegroup is tabbed open, nodes and noodles outside of the nodegroup are not editable. (It's especially hard to notice because of the darkening effect of the NodeGroup tray ... and triply so because of the inconsistent visual refreshing of the network!)

Discovered on graphicall 36912 win 64, confirmed on personal trunk/36214 win 32 no-compile-flags, confirmed on 2.57 web release win 64