Here're a few tips to drastically increase the traffic from those groups.
1. Join at least 20-50 active groups which have 100 or more members.
2. Share your knowledge at the same when you promote your business.
2. Use prominent, but not intruding color for text image in your posts. The color of the images is what gets a post noticed and clicked. Blue and a few other colors usually work well. This will make 400% difference.
3. Choose the right time to post: The CTR difference between the best time and worst time is usually 300-500%.