Hi,
I'm going to explain a little bit about the node grouping, and another message refers to this thread so I'll post it here.
The double outline shows when a node is over a group but not grouped by it. In TG2 this didn't really matter. However in TG3 there are some things which depend on nodes being in certain groups, particularly Object and Lighting groups in Render Layers, so it was felt there needed to be some way to indicate whether a node is grouped or not.
The grouping indication is not infallible. For example there may be situations where a node is grouped but shows the outline when its group is moved over another group, or as someone mentioned, multiply nested groups. We will improve this in the future.
cyphyr explained the grouping methods well. Along with the grouping methods from TG2 (capturing nodes with a group or using the Group menu command), TG3 also introduces drag and drop grouping. You can drag a node into a group to group it. The group highlights and flashes to show it will group the node if the node is dropped there. To ungroup a node simply drag it out of the group.
Regards,
Jo