There is a little button at the top of the group chain that has an icon (4 diamonds?) if you click on that you will have a large number of set-ups to choose from and I think you can make custom set ups as well where you can drag part members to specific places in the formation.
I try hard but doesn't spot any diamond icon you are referring, I know you can set party formation by pressing Esc though.
If they would implement a group select and deselect interface that would make a lot of things easier, for example, you don't need to chain party members together if you want them to follow each others, just grouping and move, the same can apply to stealth, or focus fire on a non battle object(useful breaking a door or chest).
Currently, having to break the group chain to setup tactical position before combat is somewhat inconvenient.