You would rightly deserve to be punished for doing so in my opinion even if not everyone will agree and the reason is all those groups you enter and ditch in an effort to find someone specific is screwing with all those other people in those other groups. If going to join an in progress run then help the people you first join with or don't join in the first place, the friend would probably get someone come in and help even if it is not you and if they don't then help them run it again after they finish the current run only this time help from the beginning instead of trolling all those other groups trying to find your friend.
Way too many people abuse the system while trying to fish for things like final boss only, consider yourself a casualty of war if you feel like it punishes you alongside them but while it might seem mean...I have very little sympathy for those who fish for a friends group anymore so than those who fish for final boss since I still consider it trolling a lot of other groups while searching for their friend. I have no problem with adding an invite function to an already in dungeon running group and it would be a good idea to have such but as the system works currently without it your annoying the many groups you pass through on the way to finding your specific one.