"Been dating from aged 16-20. Never been invited over for dinner or ever invited to a family event. Would joining even fix this or is it too late"
Walk away... By JW standards she's not doing right just by dating you. And that you've not been invited by her family should tell you all you need to know. People either accept you and show treat you fairly, or they do not. DO NOT ever try to do things to get another person's approval. It is not worth it. And JWs are the most conditional people. Besides that you never know how your girlfriend will become in future. What if she suddenly becomes more strict?