Master, we saw one casting out devils in Thy name; and we forbad him, because he followeth not with us. And Jesus said unto him, Forbid him not.
The disciples were confused. One who did not “belong to their group” was doing a service in Jesus’ name and they tried to stop him. This was not what Jesus wanted and so He said, “Don’t stop him.” What Jesus may also have been trying to teach the disciples (and us) is, don’t bother with what others are doing - you do your service as given by the Lord. We are each responsible to the Lord for what we do, not for what others do.