This query should do what you want. The per
value in a result row will be null
if the b
Group has no Person
belonging to it. You can play around with this query using this console link.
MATCH (:Client {value:'1234'})-[:IDENTIFIES]->(a)<-[r:PART_OF*]-(b:Group)
OPTIONAL MATCH (b)<-[:BELONGS_TO]-(per:Person)
RETURN DISTINCT a, b, per;