I want to rearrange the UV of the fractured structure to fill up the available island space. However if I use UV layout all the small pieces will scatter around, is there any way I could re-arrange them as if they are grouped or connected into one UV piece?

You can use a UV edit node. There You can move UV islands like geometry. You can select them by UV connectivity
or by group if you have set them up accordingly.

Cheers
CYTE

thanks, but I was thinking is there a way to do it procedurally?:S

Ah ok, I didn't read carefully enough. In the UV layout node under connectivity, you can specify an Island attribute.

Cheers
CYTE