So you've been granted access to another farm or consultant account.
In the mobile app on the login screen, beside the "Add an existing login?" click on the "Login here". You will login using your current credentials for your account you already have. It will take you to a "Connect as" screen where you can add the new shared account. Once you select on the account it will start to sync and download everything.
On the login screen, you will see multiple logins with the name of the accounts under each option for you to switch between each account.