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").

What made things even harder for a beginner like me, was that Synology uses custom terms and not "objectClass=user" but "objectClass=posixAccount". See:
https://www.synology.com/de-de/dsm/software_spec/directory_clients#direc...

So my final result for the filter term to get all active users from Synology NAS who are members of the group "CEO" is:
(&(objectClass=posixAccount)(memberOf=cn=CEO,cn=groups,dc=mynas,dc=local))

Where the last part "dc=mynas,dc=local" is the "Base DN" which you can copy from the configration tab of your Synology LDAP Server settings.