I've just signed up to Google Hangouts but am using a shared phone, as my friend already has an account and is logged in I don't know how to add an additional gmail address to switch accounts within the app...is this possible? If so how can I do it? Any other way I can access my Google Hangout on the phone if not?
You can add a new Google account from the main Settings app on the phone. On Samsung phones, choose the Accounts tab, then Add account and follow the steps as they come up. Once you've done this, the Hangouts app will show a spinner (drop-down box) to choose which account to use.
You might also wish to check the sync settings from the account settings. By default, when you add a new account it will also sync contacts, gmail, calendar items, &c. to the phone. If that's not what you want, you can turn off those items from the sync settings.