Don’t validate email addresses which haven’t been confirmed by the owner. In the event that someone gave an address which belongs to another user it’s a sure fire way to get flagged as spam. That could have long-term implications for your communities ability to send any emails.
It’s crucial you respect email validation and the wishes of users you have invited on to your community.
You could always export the email addresses of accounts which have yet to activate and send a one time email to them outside of Discourse to make them aware of the opportunity.