Local Viking automatically authenticates with your Google Accounts upon clicking the 'Add Account' button on your dashboard (https://www.localviking.com/google_accounts):
Once you click this button your browser will be redirected to Google and you can choose which account you want to authenticate with. Once you've granted permissions to your account, you will then be redirected to your Local Viking dashboard where you can choose which locations from the account you want to sync. The synced locations will count against your location limit.
There is currently no limit to the amount of Google accounts you can attach to your Local Viking account.