Allow an account to be the owner of one site and a member of another
I am the owner of my own tiny consulting company website. My client invited me by email to be a team member of their massive ecommerce site in cloudways but it fails because "this email is already an owner of another website". I have no idea how you could make the system this poorly designed when everything else rocks. As a consultant I am going to own my own web site and be a team member of a number of client websites (they need to be "owners" of their own sites as I may come and go from their team). I ended up having to make a second account I use to be invited to their site by a different email (yuck!). Your competitors pantheon figured this feature out (although virtually every other feature they did sucked).
Came here to say the same thing! Clients want to manage their own billing and account, but also allow a third-party to work on the server and website component. I shouldn't have to create a new email address just to be added as a member to another account. Echoing what's already been said, please make it so one email can be a member of other accounts but also the owner of their own account. One single login.