There are several places in Synapse where a user needs to reference a collaborator who they find are not yet in Synapse.
UI Mockup:
https://docs.google.com/presentation/d/1SKSzfWaOBAPdN87ivTXnLxV3ItuY7VoWANcXWeglxN0/edit?usp=sharing
create an external user token (EUT) (+ various meta data, e.g. first, last, email)
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