There is interest in how to get projects and Upwork', and then to connect to the people who are not registered on it. Could you offer on behalf of the freelancer or agency you need to register? And as he Upwork feel about that?
IMHO if the payment goes through Upwork, they should be anyway.
Tomasa answered on June 10th 19 at 16:36
Allowed, is normal, but the time for these undocumented people can be set only manually, using Manual time, and not through the tracker. Not all customers will go for it, especially if you have not pumped the profile. Well, either work on fixed-price projects, there is generally nobody cares how many people are involved.