The membership service can be used for provisioning users in the tool, however currently Moodle only seems to send the username of a user for non LTI 1.3 tools, which makes provisioning those users quite hard. It would be nice if either the username could be send as is done in the member_to_jsonld method, or if the full custom substitution parameters could be provided (this seems like what MDL-55898 was trying to achieve but it seems stuck).