What's the Best Way to Grow a Facebook Group?

Invite your friends to join the group
Join other groups in your niche and make high quality posts. You will get people who will like to be your friends and then you can add them to the group.

Find a purpose. What is the point of these groups? Once you determine that, then you will be able to attract the type of people to that group who would contribute even more to this

2 Ways

Facebook Account that has friends with the same / similar interest or niche

I created an account with friends only for online marketing. I add them, engage to them.
Posted to online marketing FB groups asking them to add me up for tips and tricks about online marketing strategies.

Add them all instantly using a script. See this tutorial: Add ALL your friends to your Facebook Group

Promote your facebook group from Youtube to get a highly targeted audience in your niche and offer them a free report or a free gift if they join your group.Rank the Youtube video or videos on the top of Youtube or Google to get traffic.

My free Kindle Book group has over 8,000 members and growing. I started by inviting my friends and actively posting 4 or 5 free books every single day. I then contacted some of the authors and asked them how they got on in the group. with this info I created a blog post with a link to the group and promoted the living daylights out of it on Twitter with a selection of hashtags.

The next step was to attract in the authors, so I wrote a mega list post of 72 places you can promote your Kindle book for free, and in the Facebook groups I listed my group at the top (declared it was my group as well if I recall correctly).

I then used Tweetadder to mass message my Twitter followers and invite them to the group if they were interested in getting free books to read... I now have authors and readers and they're connecting.

There were a few hiccups, I got an author to co-admin and he stamped out people from commenting in the group or saying they'd downloaded posts. That was a bit icky, but we got over it and we now have 6 admins to manage the group and cull the Rayban sunglasses spammers.

It took 9 months to build 2,000 member and then another 5 to reach 6,000 members. A spate of guest blogging over the last few months and leaving a link to the group in the post has helped us to get to 8,000+ members.

I hope that helps you kick start your group.

Sarah