You must have an account in each domain you to which you need access. To be made a user in a domain see Jeanne Chiang.
On the development platform:
On the staging platform: all these domains are for testing. These are not customer domains.
Domains with customer prefixes use that customer's metadata mapping and can be used for testing issues within that domain - they should not be used for general testing, and never for customization testing.
On the production server: not including customer domains; those you can find on the Customer Matrix.
Only "test" on production is to be used for testing. Do not test in customer domains. Ever.