Define which resources belong to the logged in user's group, and simply make them children of the parents they belong to. Wayfinder will show them as dropdowns if you use CSS to create the dropdown menu.
You'll need a rowTpl (usually standard from Wayfinder), wrapped by your outer ul class or Id.
Then you'll make an innerTpl like shown in the Wayfinder docs, and assign your dropdown class to that.
I posted some examples in this thread a couple days back. Hope it helps.
https://forums.modx.com/thread/84762/dropdown-menu-creation#dis-post-471006
It's really all about the CSS for the menu, and Login coupled with resource groups and permissions. The menu is the least of your worries, as once you get a menu working it'll show what you tell it to show in your resource tree hierarchy. Displaying the menu items for resource groups is a little tricky, but only because permissions can be tricky.
I'd also take a look at Bob Ray's Subscribe snippet, which is quite handy for subscribing users, and works nicely with Login.
Read through the security documentation here
http://rtfm.modx.com/display/revolution20/Security, and also read up on permissions/security at
http://bobsguides.com.