A group keeps islands together: it selects, moves and packs as one island, its islands keeping their places among them.


Select the islands, then click Add under Group, in Arrange, or press G. Any groups among the islands selected join the new one. A dashed box goes round the group in the UV Layout, named over its corner with how many islands it holds.
Select the group and click Release, or press Shift+G. Its islands go back to being separate.
Groups are kept with your work.