Yes, you can still join. Not sure if you can still do the quest (keep the report, FYI.) However, its the second-to-last quest from this one quest giver, and his quests are not required to reach the highest rank in Redoran, so...
Basically, you only did a minor screw-up that will prevent you from completing a non-required questline for the house advancement, and said quest was near the end of said non-required questline... So... You didn't really cause anything to require a restart.