Authorised User for a Separate Selling Account than my Own Selling Account Clarification

Question....I have an existing amazon seller account and I have a potential client who sent me an invitation to help them manage their account for totally different products (than mine) to help them with ppc and listing optimizations. DoI log in with my current credentials for my existing selling account or do I need to create a new account after I click on the secondary user invitation link?

Any clarification would be great.