Anna, Were you able to learn anything on this topic? I'm also pondering whether to startup an agency using a colleague that has returned to India. My concern is also taxes.
The only taxes collected by Upwork is the Value Added Tax (VAT) required by some countries. All other taxes are your responsibility to report and pay in accordance with your local laws. You may want to talk to a local legal and/or tax advisor to find out about the best ways for you to register your business and report your Upwork income to your local tax authorities.