To add a song from the media library to the queue tap the plus icon next to the song name. The icon will grey out, similar to using the On-The-Go playlist in the iPod application.
You can also add a collection of songs at once by tapping the plus icon next to a group name and selecting Add to Queue from the menu that appears.
The order in which groups of songs are added is based on the group added. For example, if an album is added the songs will be added in the order they appear on the album. If all songs by a particular artist are added they will be added in alphabetical order.