The person who is carrying the cash or equivalent means of payment (e.g. in his or her handbag, backpack, or suitcase) has the obligation to declare the whole amount in writing or notify customs orally.
This applies regardless of who is the owner of that money or payment deemed equivalent, and the purpose they carry any cash and/or equivalent means of payment for.
Example: A group of travellers is entering Germany: father, mother, and three children. The mother is carrying 40,000 euros in her handbag - money that belongs to the father and the children. Because she is carrying the whole amount, it is the mother who must declare the 40,000 euros.
It is not permitted, however, to distribute the money among a number of people in order to circumvent the obligation to declare or notify. If you do so you will be committing an offence that is punishable with a fine of up to one million euros.