The VZG chat client should be able to discover care networks in the matrix environment (space). It knows the BSN of the user and knows what organization is involved. The communication protocol should include a discovery request that appempts to discover existing spaces. Once a space is discovered, the owner of the space should send an invite.
The identity of the patient (BSN) is bound to the VGZ application space. In the matrix spec we assume that each supplier creates an own identity of the patient, if required. This results in multiple identities, bound to the supplier. So, in this case, the patient will have an VZG based identity, possibly more will exist. The client also can be available by other suppliers.