It's important to understand the terminology used as a Google Account can be linked to one or more Google services.
Google Account - This is the Email address and password that you login 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.
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.
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.