How do you add a drop down menu to your navigation menu?

On this community itself, from the homepage here:  Verint | Telligent Community if you select the 'verint community' link in the main navigation menu it opens up a drop down menu with links to what I'm guessing is sub groups for the different software versions. 

My question is, how do you make that link into a drop down? For example I have a private group that would like to use two separate 'article collections' in their group and rather than having one nav link to take you to a page where both are listed, they'd like to use a drop down affair from the main group nav instead.

However, still using the 'newbie' card here Blush, I can't for the life of me figure out how to do it?

Any hints? 

Lisa