Marketo will allow you created a new asset in the shared folder and the new asset will show in the parent folder. However, when you try to make changes the new email or landing page, Marketo will give you an error saying that you have insufficient privilege.
Note: Folders that are shared will have a little padlock on them to show that they are shared.
The reason why you are getting this error message is because shared folders are read-only. Since they are read-only, all users (including Admin users) will get the error message of insufficient privilege. If you want to make changes to the new assets, you will need to edit them in the parent folder. The parent folder will have a green arrow pointing up.