I am currently about to start work on an iOS app for somebody but they have there own Apple Developer account which they would like to us. How do I go about setting my MAC up so that I can use their account to build, develop and test their app on my Devices?
I am assuming that I will need to create a separate user account on my MAC. I have done this but when I try to install the certificates that they have created I get errors such as "Valid signing identity not found".
Why could Apple not make this process simple, I am always having to refer back to the documentation and still get confused just to start creating a new app!
You do not have to create a new user account on your Mac for this, but I know some developers prefer to have separate accounts when they are working with more than one developer account, for ease of development & distribution provisioning profiles.
If your client adds you (by Apple ID --> email address) as a team member to their account (assuming their account belongs to a company and not an individual) then you can start the development process by requesting and later receiving a signed development certificate. You'll then also need the required provisioning profile(s) to develop/distribute builds of the app.