no the groups are just used for grouping.
no groups will give the same results.
of course you could select the group AFTER growing the tree and delete it to get rid of a level.
but keep in mind, that on unbalanced trees this might not work as expected
so applying a hair material will actually let it grow along the whole tree.
a simple solution to this problem would be a modification of the plugin to add an additional spline ring for each main branch. that way unsuspecting intermediate branches wont be caught (if anyone would like this, it would be easy).
the other way to work around it will soon be available:
the tree will automatically have a decal applied that can e.g. control hair density along its lenghth.
Well, the funny thing is if I take your project in AM beta for OSX, it works without modifying anything. If I create a new project, add splines for the base and then the branch targets, and call the base "base", without the group levels, it crashes (but then OSX beta is just that). I should try it out on my crappy PC (yes, it is just crappy, not because it's a PC, although... ) and see what happens.
The extra spline sounds like an interesting idea.
Wow, the decal for density sounds cool too. Would be nice if it could be a randomly generated one.