The Paratext Registry will let you sign in with any valid email address, but if you then request a registration code as a member of a translation organisation, your organisation’s approver may expect you to be using your organisational email address, as this makes it easier to confirm your identity. It will also typically keep you from having to create and remember a password: if your organisation’s email is on Google Apps, you will see a “G” icon next to your email address and can simply sign in via Google.
If you are not a member of a translation organisation, you can use any email address. If it is not a Google/Gmail address, you will be prompted to create a password.
Paratext 7 users: You do NOT need to use the same email address that you registered with in Paratext 7. Once you are signed in, just provide your existing Registration Name and Registration Code, and your existing identity will be linked to your new Paratext 8 registration.
See also: How to register as a Paratext 8 user.