Course Paths/Branching

I've only use Storyline but I know there is a way. You'll have to be familiar with variables. In the beginning you can set up a page where the user identifies which group he/she belongs to. When they select which group it assigns a value to a variable (lets say variable "GroupNumber") . It could be a simple true/false variable.  Since the only section that is being skipped is section 3 and 4, you can assign a trigger to those slides. The trigger should be something like "skip to slide# when timeline begins, condition = if groupnumber = true".

So as soon as the  slide starts it will look at that variable and if  the variable matches then it will skip ahead.

Another option is to insert a layer in those sections. Again if the value matches then the layer pops up with a message that says "you do not have to take this section, please go to the next section". This would have to be done on the base layer as well.

Of course I don't know about them not being able to access those sections thru the sidebar menu.