Here is the way we do.
1- Log In to your Facebook Account
2- Join the niche groups you want
3- After the Group Admin confirm, you will receive notification to join that niche groups
4- Now please go into that group then copy the the Group Name from URL in address bar.
- abcd: is the Group Name
5- Combine the Group Name with @groups.facebook.com
6- Now send the Email to email@example.com
Note: Maximum 50 group (it means you can send to 50 email) per mail sending out.
7- Success with my Gmail.