If a site visitor clears cookies, uses various browsers or computers then Google Analytics will have a hard time detecting them as being by the same user.However since 95% of the site content is only available when logged in, so I should be able to identify multiple visits as the same user so long as they log in.How can I help Google to identify that the visits are by the same user? (without breaking the Terms and Conditions)
How to track site visitors across several browser sessions or computers using Google Analytics?
google analytics
If I understand your question, I think should try to send to Google Analytics the user identifier (aka User ID), as specified on their documentation:https://support.google.com/analytics/answer/3123662?hl=enThe User ID enables the association of one or more sessions (and any  activity within those sessions) with a unique and persistent ID that  you send to Analytics.To implement the User ID, you must be able to generate your own unique  IDs, consistently assign IDs to users, and include these IDs wherever  you send data to Analytics.For example, you could send the unique IDs generated by your own  authentication system to Analytics as values for the User ID. Any  engagement, like link clicks and page or screen navigation, that  happen while a unique ID is assigned can be sent and connected in  Analytics via the User ID.