What identity type are you using (Adobe ID, Enterprise ID, Federated ID), and what type of contract do you have (CC Teams/VIP Enterprise/ETLA)?
This is usually no need to resend the welcome email. The user has entitlement as soon as they have been added to the product on the Admin Console. They just need to log in with their ID to use the product.
If you still want them to get the mail you can remove them and re-add them to the product. This will work for Enterprise provided you have not chosen to turn off Notification emails for users in that product profile.