Clumped library assets
Grouping assets together in prototypes this way will greatly increase the speed at which assets are placed in the worlds, since after adding them it requires only minor tweaking of asset positions, rotations and or size.
In order to create "clumped" assets, you should first either find a nice group of assets, or create a group in your test area.
Select one of the assets, the biggest one would be most the intuitive (such as the tallest tree). This will be the parent instance. Once selected, copy its GUID from the Properties panel. Next, select all of the other assets (make sure you don't have the designated parent selected). Back in the Properties panel, paste the GUID of your designated parent into the ParentInstance field and press Enter. This will set your designated parent as the parent for all instances selected.
Then, select the parent again and open the Asset Library from the Assets panel. Find the library and tab appropriate (for groups of flora, there is a Grouped Flora tab in the Props_SpeedTree Library). Right-click in the icon area and select "Grab Selected" from the menu. This will create a prototype of the parent you set up and all of its children.
Use the "Update Icon" menu selection to give it an icon as you would any other asset.
Lastly, you'll want to name it. I suggest fairly descriptive names, as with groups of assets you may have to back off quite a bit to capture all of the assets for the icon snapshot.
Tip: A descriptive name will help people realize what types of trees they are adding.
And that's it!