I agree that the group sign should stop at the lowest/highest group in the pack rather than cycling infinitely.
Jeremy is correct in that it's not always clear from a pack author's description/naming system whether the current group is the highest or lowest, and having the up/down arrows stop at the highest/lowest groups respectively would help to make this more clear to players.
It's an easy enough fix: I've added a method to check whether the current group is the lowest or highest, and return relevant flags. It works, and can also be used for browsing through the pack from the level preview screen as well.
I'm fairly convinced this is right way to go for at least the group sign (main menu), what do people think about the arrow keys stopping at the lowest/highest groups in the level preview screen as well?