It's important to understand the terminology used as a Google Account can be linked to one or more Google services. Facebook is excluded from this conversation as it does not have the same complexity.

Google Account — This is the Email address and password that you log in to Google Services with. This can be an email address with a custom domain or Gmail address.
Google Service — This refers to the Google owned applications, such as Google Ads, Google Analytics, Google Search Console or Google My Business.

Business Accounts

Go to Admin → Connections and select the desired service. If you have previously connected a Google Account you will have the option of choosing it, or you can connect a new one. There is no restriction of how many Google Accounts you can add.

Agency Accounts

To connect Google Services to a Client, you must connect a Google Account first. Go to Admin → Connections.

Connect Google Services

If a Google Account is already active Select a Client in the Portfolio that you want to connect a Google Service, then go to Client Settings → Connections. Note that the Client Settings menu only appears if you have selected a Client from the Client selector dropdown.