LDAP query Synology NAS for all users of a group

Well some experienced LDAP administrators may laugh, but it was quite hard for me, as LDAP "newbie" to find out how to get a list of all LDAP Group Members from my Synology DiskStation. So I'd like to share my experience and result.

My task was to get a list of all active users in my Synology LDAP which belong to a certain Group ("CEO").