Zwave using groups

I’m using OH2 snapshot online version.
Whenever a node is found and added a thing, channels and (some) linked items are auto generated.
I have a couple of nodes generating temperature and I’d like to show them in the same graph.
This require me to group them together.
If I try to add a group to the item in habmin i get an error “Cannot update non-managed item”.
How should I go about to create a group for auto generated items?

Is there any documentation to understand nodes, channels and items?
Thanks in advance

You can’t edit auto generated items. I would personally recommend disabling simple mode and just create your own items…

Thanks, done that now.
Never the less. I’m unable to add that thing to my group.

I have a group specified in my .items file called "all_temp"
And when I link an item to a channel I fill in “all_temp” in the Groups field. (I end up with 2 items, one blank?)
It saves ok. But when I look again the group is gone.
Any clues?