A user can belong to multiple accounts and users can also have different roles in the various accounts that they belong to.
When logging in to the Cloud console, a user that belongs to multiple accounts will be prompted to select an account after logging in, ensuring the user is working in the correct account.
Once you've logged in, the account you are using is shown the bottom left. The account menu is also used to edit the account details.
Note: Users can have different roles in different accounts.