"active members"? I'll call bullshit. You cannot pay for members in any way other than promoting a page / group and specific content that is engaging. Want to grow your group by thousands of members? Make sure it's public. Make sure it has a "personality" to the group that is appealing to large numbers of people. Share engaging and shareable content. Every Day. For the next 3 months. Preferably twice a day.
A good strategy for me its that i post useful content that help people in other groups and i get many friendship request ,they are not so many but people come naturaly