I have a level 70 rogue that I levelled solo/duo before I started 5-boxing, and after much thinking about how to incorporate him into on of my groups, I just gave up :
He will stay at 70 until I can stick him in some of my other groups and clean instance having him doing nothing (but open boxes, obviously). For instance, when my 4 x Hunter + 1 x Druid team will be geared enough to run level 75-80 instances with a hunter missing, I will stick the rogue in the group, on follow and do a bit of instance farming to level/gear him up.