After much testing, here are a few tactics that have been proven to produce a more active group for me:
1) Every week/month, create a post to welcome your new members to your Community. This brings them back to the platform that they might have forgotten about after signing up. Your welcome post could tag new members, include who the group is for, what's in it for them, free group resources, guidelines, CTA etc.
2) Take the time to send a message to each new member within a few days after they sign up. One thing that I include with the message is a link to the "Start here" pinned post, which gives them an overview of the group and what to do next. You can also ask them a question to spark an easy conversation.
3) Post a weekly community update, which can be anything related to the Community such as the groups growth, new courses, industry news, upcoming webinars, polls etc. Just make it consistent.
4) Encourage your members to post to the community board, and let them answer each others questions. Hold back from replying to a post right away, and give your members time to respond to each other. This is a small move that can really spark engagement and bonding between members. If you have a feeling that a particular member might add value to a post, tag them and let them take it from there. Works great!
5) Encourage members to share their wins and post their goals for the week/month. This is perfect because it's easy and helps to know what your members are working on/stuck with, which can spark even more engagement. Check in on the members mid week or month and ask how they are progressing on their goals. This goes a long way!
Hope this provides some ideas for you.
-Trent