The next update alters things so you can have greater flexibility when it comes to positioning groups. You will now be able to have say three groups all in the furthest column with nothing in the column on the left of it if you wish and unlike the Windows 10 menu these groups will not get shifted when you resize the menu unless they no longer fit on the menu.
With regards to adding items, it depends how you add. If you add from right clicking on things in explorer then they will be added to the original group that Start11 initially created. Items should be inserted based on available space, so unless you have gaps in your layout they should be being added to the end of the list, but we will check that.
You can also add by dragging from the all apps list. With this you can pick where you want things exactly. The next update also adds a right click menu to let you add things to a specific group.