Once the welcome emails are sent, your staff have 28 days to use the link before it expires.  If it does expire or is lost in the email pile or gets sent to the junk folder by mistake you will want to resend the welcome email.  To do this go to the login page and first click the Validate email link followed by the 'I've forgotten my password' link.  The combination of these two links is the same as the welcome email.

To see who is still to respond to their welcome email you can go to Admin > Licences > Users (dropdown) > Outstanding email validation.

There is also a report available that shows the same information at Reports > People > Status (dropdown) > Accounts.  An absence of a date in 'Joining instructions last sent' indicates an employee who is still to validate their email from their welcome email.