You can resend a User's account activation email by going to Admin > Users and selecting the checkbox next to the User's name > Actions > Resend Invite. 

You can also click on the User's name to view their profile > Actions > Resend User Activation Invite. 

Do Account Activation Links Expire? 

Yes, and you can set your portal's invitation expiration date settings by going to  Settings > Security, Privacy, and Compliance > Security. Learn more about this topic here

How Do I Know if a User's Account Activation Link Has Expired?

The below message appears at the top of the User's profile.