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.