This specification describes apparatus comprising means for sending, from a first network function service consumer instance to an authorisation server, a request for an access token for use in accessing a service provided by a network function service producer, wherein the request includes an identifier of a set of network function service consumer instances of which the first network function service consumer instance is a member; receiving, at the first network function service consumer instance from the authorisation server, an access token for use in accessing the service provided by the network function service producer, wherein the access token includes the identifier of the set of network function service consumer instances of which the first network function service consumer instance is a member; and sending, from the first network function service consumer instance to the network function service producer, a request to access the service provided by the network function service producer, wherein the request to access the service includes the access token.