The best way to engage with your audience is to offer them something they will want and, at the same time, it will make money to you.
For example and, taking what
@john8874 says, a CPA offer that will work amazing it´s a giveaway or sweepstake type of offer. But, you WILL DELIVER THE PRIZE IN THE END!
The difference here is that, you will be offering, let´s say a year's worth of Shopify ($360 approx, right) giveaway for anyone to enter the giveaway, like/share, and comment the post.
Any email submit offer pays anywhere between $1 to $3 but, you don´t care about the actual prize, you can offer it as a "bonus" but the main prize is the full paid yeat on Shopify.
You only need 200-300 people to sign in (10% of your entire group) + any other that comes in after.
You will get huge engagement from this and get some or ALL the money to cover this prize from the CPA offer.
Damn, you can also enter Shopify Affiliate Program and get some value from there as well.