Usually people can use a Facebook Graph Search URL like https://www.facebook.com/search/2204510018/members/2204420210/members/intersect or https://www.facebook.com/search/str/linux/groups-named/members/str/surfing/groups-named/members/intersect to find people who are in several groups at the same time. One day, I noticed that the URL was not working anymore (it only worked in the mobile version m.facebook.com or in another Facebook account), so I created a script using the Facebook Graph API to display the groups members intersection.
You can find a Facebook group ID in the group URL or in the “Report Group” link.
At the moment, it looks like people have to use Graph Search URLs instead of Graph API to find group members. Other Graph Search URLs can be found here: