It strongly depends on the purpose of your group and the value you give to its subscribers. Right now the best way to achieve organic growth of your group is to use all other channels besides FB, which would have a link to your group (they are different in each industry/country/etc.). 1 suggestion: don't use any services, which would add a lot of non-relevant users to your group, this would destroy its ability to deliver organic traffic from your posts to the real users of your group.