Authorised agent for joint authors in publication agreement For the publication of a paper each of the joint authors needs to sign a publication agreement with the publisher. Instead, only one author can sign as an 'authorised agent for joint authors'. Ignoring the specifics of this particular publisher, how does an author become an authorised agent generally in this respect?
They're taking the word of the person who signs, that they're allowed to speak for everyone. 

Or, in some contexts, the person is authorized by their own institution. For instance, a VP of a company signing for all employees who were authors,